The motor mount will take an N20 motor.  I used a 6volt, 300rpm unit but other voltage and speed ratios are available.  You will also require two M3 threaded brass inserts, melted into the mount for securing to the floor of the vehicle.  The axles (from Peter Korzelius wagon wheelsets) need two m5 x m8 x M3 ball races inserted into the shaft housing of the mount.  A dab of adhesive helps to secure these in place.  A spur gear drive gives a 2:1 reduction and can be created from the following files.