Well i read the rules and maybe missed some things...
Why not put a turbine to power the wheels?
I had a chance to check out the canisters, drew the following conclustions:
With an exit OD of 6mm and a wall thickness of 1.5mm, that leaves a 3mm ID.
So using a revised expansion ratio (Pi=48MPa, Pe=95KPa, k=1.289 CO2) of 129 the exit needs to be 34mm diameter
Hope this helps, now make it fast already!