If I bought oversized valves for my warrior would I have to have the head modified or anything?

 

Yes. Enlarged seats and ported passages, oh my!

Before taking that step, piping, camming (increased lift/duration/overlap), overboring (stroking, too, if you've got the dough) and raising the compression ratio, as well as maybe a larger carburetor would be in order. Increasing the valve and seat size alone won't give much additional performance from an otherwise stock engine. Depending upon the head geometry, you might not have much valve diameter expansion possible, before the seats begin encroaching on each other.

Tree Farmer