If you were to put the motor on the right side of the car, you would have to get it to rotate counter-clockwise, to match the rotation of the tires/belts.

Stock motors are built with timing in them - American stock motors are 24 degrees, I don't know what others use, but it may be varying amounts, still the same direction.

If you're running the motor forward (clockwise), this 24 degrees is advanced timing. If you're running the motor in reverse (counter-clockwise), it would be retarded timing. This would be very hard on brushes and coms and would be very inefficient and would not have very good performance.