[Cliffradical]

Honestly, if you're happy with the performance as it is, run it till it dies.
Just bust the box right out. You *can* gut and replace everything in there, but there's really no sense doing it if you're happy with it as is.

You should probably modify or replace that hopup unit though.

Edit:
Here's why;
Nearly any upgrade part will be better than stock, but aftermarket parts don't make a good gun. Aftermarket parts where conscientiously and correctly applied make a good gun. A lot of upgrades (especially it seems within LMGs) are chasing dragons. Tilting at windmills even.
If your gun runs well enough out of the box, and/ or you adpt your play style to what it can accomplish, your gun is already a good gun.
Your stock parts may fail sooner than fancy ones, but if they haven't failed yet, they go into the tickle trunk and are never used again. That's a net waste of your money and materials.

You can benefit from taking your existing parts and correctly applying them. Gut the gearbox back to the outer walls. Get everything clean to an eye-rollingly anal standard. Re-shim and re-lube everything with fancypants lubricant (not too much!). Correct for angle of engagement. Reassemble everything and lubricate the charging handle, spring, and guide rod for it if you want to feel through.

With this done, the stock gun you're already happy with could last years and years without a failure.
Start a swear jar for the day it goes "GRRRRNK", and replace the innards top to bottom with the finest snake oil, happy in the knowledge that you earned it.