What I do for a mild to wild setup is very simple.
mild-stock valves with full port work
medium-larger then stock valves with full port work
wild-largest valves that would fit will full port work
I currently run a medium setup on my gasoline minibike. 27mm intake(2mm larger) valve with a stock size exhaust valve. I have done other heads with larger valves then that. The porting I do is basically the same on all of them.