Forget it. Even if you could upgrade the motor, the chassis is so heavy it's just not worth it. Even the Polaris Scramblers, which have much bigger engines, are horrible because it's the same chassis. Which is also why the Trailblazer chassis is SO heavy and over-built for a little 20 horse 250cc engine, because the chassis was designed for the 500CC Scrambler.
They make exhaust kits, you'll pick up some horsepower. But it's still a heavy chassis and under-powered engine. I'd keep it all stock (they're more valuable that way) and sell it and buy a 300EX or something similar when you're really ready to upgrade.
Duncan Racing 340PV, +2 A-arms, +4 LSR axle, and a host of other goodies.