Check out this site, if you want to try and keep up with your friends, it will take alot of work, but these guys can help you not fall too far behind. For $830, you can get a whole new worked over top end, they will even install the new cam and set the valve lash, when you get it all you will need to do is bolt it on. You will probably need a good aftermarket exhuast and a bigger carb to go with it, you will probably be into it for $1000 plus. If you just want to do the piston, they can do that for you too, they will bore & hone the cylinder, and send you the piston and a top end gasket set for $270. There are also several other kits for your Warrior, check them out and see what fits in your budget. Yamaha Warrior performance kit.
Yours truly, sincerelly, love always, Josh aka The DirtDemon.