Short answer: no.
Long answer: Nearly all cheapo guns can't be upgraded in many ways as sometimes the parts are proprietary. As well as that, when opening the gun up you could potentially break some of the internals as they would probably be pretty cheap and fragile.
TBH something as cheap and plasticy as the D68 isn't worth it in the long term, an actual gearbox to replace the original would cost almost the same as a new D68.
If you did go ahead and replace every part with an actual component from manufacturers such as ASG, Prometheus, MadBull, Lonex etc. you would have probably have spent enough to buy a new G&G Combat Machine.
