Shawn:

We had good luck with race .21 (high RPM) with good results on our project CT4. .21 will be best for this project. .12 does not have enough horsepower and .18 usually have less RPM. However, the epic .18 does sound like a good choice without spending top dollars. We use the stock spurs but drop down the pinions.

I think new era models make a .21 convesion kit.