AFAIK, the stuff need to be change is the blow-back chamber, innerbarrel-hopup assembly. The rest are usable. KWAUSA sells these stuff just need to phone in
as for non-s7 mags, look for Chinese clone of KSC USP. KSC sold their old molds to one of chinese company
This is one:
http://new.rsov.com/product.php?lang...=3653&cateId=0