Any ideas how to optimize this batch from a third party?

With only an estimated plan, I can only have so much insight into exactly how these things will help, but I do have several ideas. I tried to comment all of my changes inline, and I'm sorry if there are syntax errors (I couldn't really validate without schema). I am also pretty sure you can do this without a cursor, but I only have so much time I can spend on a Sunday night reverse engineering your code. :-)

 CREATE PROCEDURE dbo.spResRapTeoretiskKonsern
   @Aarstall  INT, 
   @Maned     INT, 
   @Konsern   VARCHAR(10), 
   @Avdeling  VARCHAR(50), 
   @Region    VARCHAR(10), 
   @Distrikt  VARCHAR(10)
 AS
 BEGIN
   -- always use SET NOCOUNT ON
   SET NOCOUNT ON;
 
   -- why not just say DECLARE once?
   DECLARE @DataAreaID              VARCHAR(50),
           @SumInntekter            FLOAT,
           @Varekjop                FLOAT,
           @VarekjopTeoretisk       FLOAT,
           @Driftskostnader         FLOAT,
           @Driftsresultat          FLOAT,
           @NettoFinans             FLOAT,
           @ResultatSkatt           FLOAT,
           @OrdinaertResultat       FLOAT,
           @AarsResultat            FLOAT,
           @VirkeligVarebeholdning  FLOAT,
           @TeoretiskVareBeholdning FLOAT,
           @VarelagerEndring        FLOAT,
           @FirmaNavn               VARCHAR(50);
     
     CREATE TABLE #TMP_rsProfitAndLoss 
     (
       Sorting         VARCHAR(10)
     , AccountName     VARCHAR(100)
     , AccountNum      VARCHAR(50)
     , DataAreaId      VARCHAR(50)
     , RsReportGroup   VARCHAR(60)
     , Transdate       DATETIME
     , Prosentsats     FLOAT
     , Sum1            FLOAT
     , ReportID        VARCHAR(50)
     , Salgsinntekter1 FLOAT
     , Prosent1        FLOAT
     , Konsern         VARCHAR(10)
     , Firmanavn       VARCHAR(50)
     ); -- always use semi-colons; see
     -- http://sqlblog.com/blogs/aaron_bertrand/archive/2009/09/03/ladies-and-gentlemen-start-your-semi-colons.aspx
     
     -- get rid of the IF conditionals. 
     -- you can just change the DIMENSION = clause slightly to support wildcard
     -- (or you can run the SELECT using dynamic SQL to avoid confusing the optimizer)
 
     INSERT INTO #TMP_rsProfitAndLoss
     SELECT SORTING, LEDGERTABLEACCOUNTNAME, LEDGERTABLEACCOUNTNUM, [DATAAREAID#5], 
       REPORTGROUPNAME, TRANSDATE,
       (
         -- this TOP 1 without ORDER BY seems fishy to me
         -- I am sure you could make this a proper join
         SELECT TOP 1 (Case when (percent_ is null) then 0 else percent_ end) 
         FROM dbo.IKLRSTHEORETICALGROSSPROFIT -- always use schema prefixes, see
           -- http://sqlblog.com/blogs/aaron_bertrand/archive/2009/10/11/bad-habits-to-kick-avoiding-the-schema-prefix.aspx
         WHERE companyid = p.[Dataareaid#5] AND dimension = ''
         AND p.transdate >= FromDate AND p.transdate <= ToDate)
       ) AS Prosentsats,
       (Case when (year(transdate)=@Aarstall AND month(transdate)<=@Maned) then amountmst*-1 else 0 end) AS Sum1,
       ReportID, '', 0, c.IMPORTVATNUMBRANCHID, c.ADBLEDCOMPANYNAMEMARKET
       FROM dbo.RSPROFITANDLOSS AS p
           JOIN dbo.COMPANYINFO  AS c 
         ON p.[DATAAREAID#5] = c.DATAAREAID
         -- use table aliases and specify which table these columns come from!
       WHERE ReportID = 'KRES' AND (year(transdate)=@Aarstall AND month(transdate)<=@Maned) 
       AND (c.IMPORTVATNUMBRANCHID = @Konsern) 
       AND DIMENSION LIKE @Avdeling 
       AND iklCompanyRegionId LIKE @Region 
       AND iklCompanyDistrictId LIKE @Distrikt
       ORDER BY LEDGERTABLEACCOUNTNUM;
 
     -- include sum1 in the index to avoid RID lookups
     -- also note that DataAreaId should be the leading column
     CREATE INDEX IX_1 on #TMP_rsProfitAndLoss (DataAreaId, Sorting) INCLUDE(sum1);
 
     -- alternatively, make this a clustered index
     -- CREATE CLUSTERED INDEX IX_1 ON #TMP_rsProfitAndLass(DataAreaId, Sorting);
 
     -- always use LOCAL FAST_FORWARD on firehose cursors; see
     -- http://www.sqlperformance.com/2012/09/t-sql-queries/cursor-options
 
     DECLARE DataAreAId_cursor CURSOR LOCAL FAST_FORWARD
     FOR
         SELECT DISTINCT DataAreaId
         FROM #TMP_rsProfitAndLoss;
 
     OPEN DataAreAId_cursor;
     
     FETCH NEXT FROM DataAreAId_cursor INTO @DataAreaID
     WHILE (@@FETCH_STATUS <> -1)
     BEGIN
         IF (@@FETCH_STATUS <> -2)
         BEGIN    
           -- combine these to only perform one scan
             SELECT 
               @SumInntekter      = SUM(CASE WHEN Sorting = '13'  THEN Sum1 END),
               @Driftskostnader   = SUM(CASE WHEN Sorting = '29'  THEN Sum1 END),
               @NettoFinans       = SUM(CASE WHEN Sorting = '45'  THEN Sum1 END), 
               @Varekjop          = SUM(CASE WHEN Sorting = '21'  THEN Sum1 END),
               @VarekjopTeoretisk = SUM(CASE WHEN Sorting = '11'  THEN ((Sum1 - (Sum1 * Prosentsats)/100) * -1) END),
               @OrdinaertResultat = SUM(CASE WHEN Sorting = '47'  THEN Sum1 END),
               @AarsResultat      = SUM(CASE WHEN Sorting = '200' THEN Sum1 END),
               @FirmaNavn         = MAX(FirmaNavn) -- making a guess here that MAX will be acceptable
             FROM #TMP_rsProfitAndLoss WHERE DataAreaID = @DataAreaID;
     
             IF (@VarekjopTeoretisk <> 0)
             BEGIN
                 SET @Driftskostnader = (@Driftskostnader + @VarekjopTeoretisk - @Varekjop);
                 SET @Driftsresultat = (@SumInntekter + @Driftskostnader);
                 SET @ResultatSkatt = (@Driftsresultat + @NettoFinans);
     
                 -- we can do this without verbose IF logic and without checking
                 -- for an empty string - not sure how a SUM could yield '':
                 SELECT @OrdinaertResultat = COALESCE(@OrdinaertResultat, 0),
                        @AarsResultat = COALESCE(@AarsResultat, 0);
 
                 SELECT @OrdinaertResultat += @ResultatSkatt,
                        @AarsResultat += @OrdinaertResultat;
     
                 -- we can also combine these into a single statement:
 
                 DELETE #TMP_rsProfitAndLoss WHERE DataAreaID = @DataAreaID
                               AND SORTING IN ('21','29','30','46','48','49');
     
         SELECT @VirkeligVarebeholdning = SUM(amountmst) 
           FROM dbo.ledgertrans WHERE (accountnum = '1460' 
           AND year(transdate)=@Aarstall AND month(transdate)<=@Maned AND DATAAREAID = @DataAreaID);
     
         --For å sikre at det kommer ut tall på de uten teoretiske beholdninger også.
         if @VirkeligVarebeholdning = '' SET @VirkeligVarebeholdning = 0;
         if @Varekjop = '' SET @Varekjop = 0;
         if @VarekjopTeoretisk = '' SET @VarekjopTeoretisk = 0;
     
         SET @TeoretiskVareBeholdning = @VirkeligVarebeholdning - @Varekjop + @VarekjopTeoretisk;
         SET @VarelagerEndring = @TeoretiskVareBeholdning - @VirkeligVarebeholdning;
     
           INSERT INTO #TMP_rsProfitAndLoss
           (SORTING, AccountName, AccountNum, RsReportGroup, Firmanavn, Sum1, DataAreaID)
           VALUES
           ('21', 'Varekostnad', '0000', 'Varekostnad', @FirmaNavn, 0, @DataAreaID),
           ('29', 'SUM DRIFTSKOSTNADER', '0000', 'SUM DRIFTSKOSTNADER', @FirmaNavn, @Driftskostnader, @DataAreaID),
           ('30', 'DRIFTSRESULTAT', '0000', 'DRIFTSRESULTAT', @FirmaNavn, @Driftsresultat, @DataAreaID),
           ('46', 'RESULTAT FØR SKATT', '0000', 'RESULTAT FØR SKATT', @FirmaNavn, @ResultatSkatt, @DataAreaID),
           ('48', 'ORINÆRT RESULTAT', '0000', 'ORINÆRT RESULTAT', @FirmaNavn, @OrdinaertResultat, @DataAreaID),
           ('49', 'ÅRSRESULTAT', '0000', 'ÅRSRESULTAT', @FirmaNavn, @AarsResultat, @DataAreaID),
           ('22', 'Varekostnad Teoretisk', '0000', 'Varekostnad Teoretisk', @FirmaNavn, @VarekjopTeoretisk, @DataAreaID),
           ('97', 'Teoretisk varebeholdning', '0000', 'Teoretisk varebeholdning', @FirmaNavn, @TeoretiskVareBeholdning, @DataAreaID),
           ('98', 'Varebeholdning ved siste telling', '0000', 'Varebeholdning ved siste telling', @FirmaNavn, @VirkeligVarebeholdning, @DataAreaID),
           ('99', 'Varelager endring', '0000', 'Varelager endring', @FirmaNavn, @VarelagerEndring, @DataAreaID);
         END
         FETCH NEXT FROM DataAreAId_cursor INTO @DataAreaID;
     END
     DELETE #TMP_rsProfitAndLoss WHERE SORTING = '200'; --Brukes kun for å bygge opp rapporten
     
     CLOSE DataAreAId_cursor;
     DEALLOCATE DataAreAId_cursor;
     
     SELECT * FROM #TMP_rsProfitAndLoss 
       ORDER BY DataAreaId, Sorting;
 
     DROP TABLE #TMP_rsProfitAndLoss;
 END