I blew my motor.
Im starting to rebuild it.
I want to make it keep up with the 450's.
uhm, good luck. You're best bet would be a nice 426/440cc kit. Some have even made 416s hang with mild 450's.
A 440 will be the least reliable and you will have to upgrade to tougher head studs(GT thunder)
Duncan racing makes a nice kit that uses its own cylinder/piston combo.
That is if you wanna go 440 and above, but it'll have questionable reliablility. Especially if you plan to desert race.
Good luck with it, those bikes are fun, they just lack a bit in the power department these days.