It's a timing issue. I'm not sure how to solve it at this point. The motor runs with advanced timing in one direction, and retarded timing in the other. I've been seeing the same behavior in other Mini-Z motors as well. Usually the timing is adjusted by rotating the endbell (and brushes) relative to the motor can.
Here's a thought...
Maybe the motor shaft could be forced out through the end bell, and the pinion mounted on the endbell side instead, which would allow the motor to run in the proper direction. The mounts would need to be revised in order to make this work.