RE: RC Lawnmower
i remember them doing it on "Scrapheap challenge" on time (TV show, similar to Junkyard wars) they used a compressed gas cylinder to work the shifter (it was automatic, but they used it to switch into Drive, Neutral and Reverse), a wheelchair wheel attached to the steering wheel to turn and i forget what they used for the accelerator pedal...
could be done though, if you have the tools and materials