Sub 300 FPS? Pretty easy.
Gears:
If you want good ROF, some SHS/Super Shooter 13:1 ratio gears will give you the desired effect, due to the low FPS spring, it's not too stressful. SHS are a great brand for internals. So strong.
Or you can go for a more DMR set of gears which are made to stand up to 450fps. As long as it has the correct number of teeth. Just one too many will be an issue! (Because guns like SR25's often have specially long pistons to accommodate a bigger air pocket, therefore have more teeth on a sector gear).
Piston:
SHS 3 steel tooth piston. Simples. Got these in all my guns. Best durability since duracell wore a crash helmet. Also see if you can grab an Element Silent Piston head... they work funnily enough.
Shimmings & Bearings:
SHS or Systema shimmings. Get a correct amount on the gears and the gun will be quieter, more efficient and longer lasting.
7mm or 8mm steel bearings are always good. Just makes your gun more efficient over all.
The rest... the cylinder, cylinder head, spring guide, spring (unless fps is not desired), trigger mech, wiring, GB casing and motor are optional. TBH, if it ain't broke, don't fix it!