This looks like a really interesting project, looking foward to seeing it progress.
What size motor are you using at the moment? 540? If you go up to 600 size, there are some very torquey ones available for not much money. Look at 'Graupner Speed 600' motors (for R/C cars / trucks / boats) - these have a shed load of torque and cost me less than £10 each. Not sure what power you're feeding to yours, but the Graupner one has two versions - 9.6V or 7.2V.