You can make anything you want as fast as you want if you have enough money, simple as that. Cheapest one to work with is the banshee.

 

You can't beat the banshee with all the aftermarket offerings. Probably a dozen companies offer aftermarket cylinders for the banshee (more than offer them for the 250R). As 14mins said, you never hear of a superfast LT500. The cases won't hold the power the banshees will.

 

i remember when i was like 4 before i knew what 4-wheeler really was i saw a lt500 and banshee racing down my street and i thought it was cool, now i ride with those guys and they said back in the day the lt ruled until they became extinct

 

take a look at the pro bikes and you'll find lots of cr500 engines and the like crammed into 250r chassis. this question is way too open ended to have even an engine answer.

 

weld a jet fighter engine to a quad and, if it stays on the ground or even right side up, youll have the fastest quad ever.

btw, a C5 Z06 corvette will beat a blue angel jet fighter in 0-60 and quarter mile times.