After removing the starter motor from the engine it was tested on the bench and found to be completely dead. Having stripped it, both the brushes and commutator were found to be worn out. To rectify this the commutator was turned down in the lathe undercut between the segments and polished with fine emery cloth. Luckily there was plenty of copper in the segments and it turned up nicely parallel. After thoroughly cleaning the brush plate it was fitted with new brushes and the bearings greased. At this point the casing was cleaned and during the cleaning process one of the permanent magnets fell out and had to be reattached using epoxy resin. Once this had cured the motor was reassembled, tested and refitted to the quad. Job Done!