I've thought long and hard about the drill option, and they just don't seem to cut it. With 18cm dia wheels, they will go at a completely uncontrolable speed, and although this is easily sorted with spektrum gear, there is the torque issue: there will be no chance of getting under opponents, let alone pushing them around. One idea I had was to put in further stages of gearing, and I am looking into that. My main problem is my severe lack of experience with creating my own gearing. I wouldn't know what to do with a tensioner if you stuck it under my nose . If someone who knows what they are doing could explain how to do extra gearing to me, I would be very grateful.

Thank you to everyone who has offered their help so far.