ok Im not sure as to the gearbox arrangement, but I would imagen its the standard gearing.

Paul you should see 2 gears, one sat on top with one gear that splits the 2(this sits over the motor), you should be able to remove the gear above the motor and join the two gears one on top of each other, I do this by putting a thin bit of metal down the thick bit of plastic on the gear on top and use some glue just to help keep them together

hope that sheds some light