Why don't you rip the motor out of the sport and buy a polaris snowmobile engine maybe a little 440 twin or something like that anyway you would defianetly waste any banshee's or 250R's oh yeah and if you don't think they don't make much horsepower can you say around 90HP.
Anyway I have no idea if this would work but it would rip.

BUB

 

I saw some pretty good polaris mxers at the track.......they raced pro cass.....that had aftermarket frames cause they had double a-arms and everything.....they seemed pretty good til one started fallin apart seat came off.....it was kinda funny......

 

I say get out and try it stock! Get alot of practice and go for it and have fun! I like to see guys getting into racing without the high doller equipment. In fact I think the classes should be set up so people without the money are the only ones who are competitive. To me it is just like radar running on a snowmobile. The rider with the most money wins! I know racing a quad takes way more skill than radar runs so lets keep racing a skill event not a money event!