Rotax kart 125 2-stroke in 120?

While I generally agree that more power is always better, the 120 is already very snappy and I think has 14hp. Adding much more power would probably render the other aspects of the machine inadequate and thus the ridiculously expensive build up begins.

However, replacing a 60's engine with a 120 or one of these 125's might be a worth while endeavor. What would one of these cost, complete with clutch in the 15-20hp range?

Opps! Just realized I linked into the youth section and you were referring to a zr120. I was thinking of the Snowhawk 120.