I have a 2001 Yerf Dog 90cc 2 stroke atv that says monsoon motorsports on the vin tag so I believe that's who makes it. I was looking to swap the engine for something 4 stroke with some more torque as we use it as a pit bike for adults most of the time I am just looking for some guidance on what may be close to a direct fit i think it is a gy6 style motor any help is appreciated.

 

Welcome to the forums.
I've made several "adult pit bikes". That 2 stroke can be made into a mighty fun pit ride.
I can't see a gain sticking in a 4 stroke slow poke...