If I were you, instead of boring it and spending a lot of money, I would spend it on a new quad. I'm not trying to rip on the warrior, they are good bikes, but if you want more power and better handling too I think you are better off with something else.
If you still want to keep it, get a pipe and airbox mods, then high compression piston and cam

 

porto, 90mm piston kit with a 71.5mm stroker crank will get you 454cc, lots more torque and if you do up the head so it will flow, the horsepower will go up in the revs.