Here is a part break down of the transmission in my rc monster truck. The 2 speed in my truck is interal and is not outside like most rc cars. This is mainly because they need to stay clean...
I am not sure how you could make one to work because they are very high speed. I believe the engine in my rc car spins at around 50,000 RPM!
2 speed
My Tmaxx truck
I am not sure if this will help you or not but this will give you a better understanding of how a 2 speed forward/reverse nitro transmission works.
There is also a 2 speed Emaxx truck that uses a servo to shift between gears I have never accually look at one but that might be something you want to look at!
http://www.traxxas.com/products/elec...s-exploded.pdf