Virtually anything can be done with enough time, money, skill and the appropriate tools. That being said... if you haven't got a bunch of experience modifying PCs, the chances of you being able to put a G4 in a G5 case or liquid cooling it, let alone building your own case from scratch, are not very high.
I'm not looking to discourage you at all, but outside of liquid cooling it, none of these projects are going to be quick, easy jobs.