I've been tempting on building another. I had a good 4 or 6 since 2009. I used to do smiley face burnouts all over, and took it on 2 full county trips many times.
The only suggestion I have is find a multi speed or a transmission with reverse like a tmaxx and take out reverse and make it a neutral.
If you make one, I would go similar to a motorized bicycle. Cable pulls a lever, lever turns a rod that basically puts or releases tension on a spring.