It would seem the hardest part of this particular job will be getting the correct dimensions since the OP's springs are both missing. That makes the job of making a couple from spring stock much harder.
Simson made quite a few guns. I'd say the best place to start would be to try to find at least one used mainspring for that particular model. Certainly a pair would be better. Start by checking Numrich- Gun Parts corp., and other gun parts sellers, check Ebay, and try to find European gun parts dealers. A gunsmith that specializes in German shotguns might have spare parts.
