well apart from the gearbox being the aforementioned couple of screws the barrel pops out with the same quick release mechanism as the real m60 (kinda nifty tbh) which gives you access to the hop unit.
that's pretty much all you need to service/tinker with the gun, after that it's a case of just unscrewing things as and when you see them.
been a while since i tinkered with an m60 so can't really help with much more specific than that.