How are the Axial motors. I heard that they are really good especially the R models. I had a R1 .12 but never ran it since I sold the car. I was quite impressed with the design. Even the exhaust ports going out of the exhaust header were ported and modded compared to other engines I've owned.
It should be fairly universal as long as it's not a mid-block but a true .21. Also you want to stick with the off-road motors since you need more torque on the low and mid compared to on-road motors that have high rpm and the power is more on the top end of the powerband. There's quite a few...