The gearbox just lifts off the motor. To reassemble, make sure the gap in the middle of the 3 nylon gears is as large as possible otherwise it will not be possible to insert the gear on the end of the motor shaft between them.

The steel disk and the gear on the motor shaft can simply be lifted off.

The black plastic piece can be removed from the motor by undoing 2 screws.

A replacement motor can be found online from Far Eastern sellers by searching for the part number RS380S-3.6V. Select one with a 6mm shaft.