Vepr12 or Saiga12? I prefer Saiga.
Both guns need to be modify.
Boltcatch? Why? Fine loading is with open bolt and you can modify safety.
Weaver? Which one? I like my C-more behind gasblock. Weaver over extraction window sometimes kick back empty shell.
Magwell? Same battle as M16 vs. AK47. If you want to, you can buy aftermarket part and put it on(someone makes it from aluminium).
All the rest of adjustment is all about your wallet and design.
It's neverending story and i'm saigaholic...
BTW - prepare yourself for changing bolt. It will broke, potentialy...