basically ur gearbox is gonna need pretty much all metal internals:
I'd install metal bushings, metal gears/torque up gears, metal antireversal latch (if it isn't already), stainless piston head, maybe upgrade the cyllinder if u think it needs it, ballbearing spring guide. Im not sure from ur description if its a metal gearbox itself but if it isn't ur probs gonna crack it within the first few shots if ur upgrading to a heavier duty spring.
ALSO: having installed any new gear upgrades make absolutely sure it is properly lubed and shimmed or ur gun is in for explosive decompression lol - or at least ur gonna cause some major damage :S
I'm not an expert at this so feel free to correct me if anything i said was wrong but thats how I'd do things
