[rc13]

If I was to mount up the motors, with the round motor mounting plate thickness in combo with the slight thickness of the sidewall of the dozer chassis, by the time I get the gearbox mounted then the spur gear on the motor is more recessed, therefore, the gear mesh would only be about half (so in a nutshell, the half of the motor gear would only be meshing with half of the gearbox gear)...

The newer motor / gearbox is to mount on the outside of the gearbox housing. The original motor/gearbox for this dozer mounts between the motor and gearbox housing.

Also the sprocket gear for the tracks has a large hole that has a locking pin that slides through the motor shaft. The collar that is between the gearbox and the shaft of the new motor is too big to fit inside of the hole of the sprocket.