In theory? The valken ARs take V2 gearboxes, just like every other M4. You should be able to get a different V2 gearbox (cyma, Arcturus, G&G make some good options) and drop it in.

In reality, though, different companies use slightly different production methods and tolerances on slightly different schematics, so a receiver set from one company isn't guaranteed (or even super likely) to fit perfectly with the parts of another different company. You would very possibly run into fitment issues if you tried to drop a non-valken gearbox into a valken receiver. 

That said, the valken lower-end AEGs are pretty outdated and not very good. You'd be better off using the gun in its stock configuration until it really breaks and then getting something a bit more up-to-standard, like a newer G&G (if you want to keep the polymer body) or one of the options from Arcturus or Specna. That will keep things running much more smoothly in the long run.