Normally with Honda utility quads, I agree with retro in that trying to add hp is usually a fruitless endeavor.
But the trx90 is a different deal. That's because there is youth atv racing and there are tons of mods available for it, and since it is legit racing and the racing parents aren't afraid to spend big $$$, you can get some serious hp gains. Like getting 20 hp or more our of big bore kits combined with race heads and intake mods. But all those gains come at huge $$$. Way more cost effective to buy a bigger displacement atv.
TRX90 114cc bore kit - anyone ridden one, I want a ride report please ?
Tons of other links, videos, and pruduct websites out there on high hp youth honda's as well.
But if you really want more hp out of the 90, you can get it. Another part of the reason is it shares the same motor platform as the crf70 and other pit bikes, and the guys that do pit bike racing also do the same serious mods.
All that to say I would never do that myself, there are much less expensive ways to get a 20hp atv that a kid can ride. But did want you to know that the mods are not just limited to 10% gains...