type 97 upgrades

[Forbbiden]

so got a type 97, and I'm thinking of getting some upgrades for it. but it's a custom gearbox i was wodnering if anyone knows what parts are compatible.

looking online the only upgradeable parts i've found are the barrel, piston,spring guide, piston head.

also for some weird reason recently when ever i use single shot, after about 7-10 shots it stops firing in single and i need to switch to auto, pull the trigger and then switch to single again for it to work.