SCS used to have all registered users software code numbers available in the SCS forum's site (My Software). I don't know if Magix will do the same. If it's had been registrered in your name, they probably have it. Contact customer service.. but that might be another roadblock since it's not listed AFAIK.
update: as Wolfgang stated, your regisered SCS software code #s are curently still on the 'My Software' page.
At the moment that function is still located at the SCS side. Was the software registrated there? Then you should be able to find the SN in your account. Otherwise ist is gone ...
If you did not registered your software with SONY then and now you lost the serial#. Its pretty much is gone for good. If you have registered, you certainly can retrive it.