it looks like the only difference in the 1 speed car and the 2 speed is the gear set. so...
as far as i know you could purchase the 2 speed moderate gear set for 26 bones here
http://www.redcatracing.com/vehicle_...m?vehicle_id=2 and it should bolt right in... personaly i would try sence it is only 26 bones, but your might want to ask around the 1/10 truck forum to make sure its that simple, sence there are more people on the truck forum than the buggy and they are the same driveline setup. Realy i dont think any one has tried a 1 into 2 swap with a redcat, i does not hurt to try. good luck man!!!