I would do the following:
Metal, bearing spring guide. Despite having a V7 gearbox, the CYMA M14s actually use a V3 spring guide, so take care to get the right one.
Guarder SP130 or M140 spring.
Sorbo on the cylinder head (70D). This has three functions. Firstlly, to correct the angle of engagement between the piston and sector gear teeth, secondly to act as a shock absorber to help the gearbox to handle the impact from a stiff spring, and thirdly to quiet the gun a bit (although it honestly doesn't do all that much, in that regard, but it does change the sound from a sharp "clack" to more of a dull "thud").
ACM high-torque motor such as
this or G&P M160.
Prommy piston head
The stock piston should be fine, but if you really want to play it safe, get a Prommy.
Get a new air nozzle, one with an o-ring. I've had great luck with guarder.
Correct any other compression issues. If at the cylinder head, you can seal with silicon sealant. At the piston head, either replace or stretch the o-ring.
Polishing the cylinder wouldn't be a bad idea.
Obviously re-shim and re-grease.
A mosfet wouldn't be a bad idea, especially if you plan to use a lipo (which would really help your trigger response since those neo-magnet motors like a lot of current).
Everyone seems to have their own favorite hop-up bucking/nub and barrel combinations, use whatever gets you results.
That's how I'd do it at least.
