I fitted a Motobatt , Valeo knock off from Caltric on Ebay and HD battery leads at the same time, 8 years/ 200,000 km back .
Total cost was around 40% of the cost of a Nipon Denso starte alone, at that time.
There is a BMW tech bulletin advising to change the starter relay if you change from a Bosch to a Valeo, which I followed ,err, later!
Thenew bits transformed the starting from reluctant to instant and spin the bike over like a sewing machine and they are all still performing like new, on my hi comp 1000 cc G/S.
Kickback from a wrongly timed or misfiring ignition is usually the cause of stripped planetary gears.
The plastic gears on the OE French Valeo on my GS stripped when a tooth broke off the starter ring and jammed the starter.
Replacement gears for the later Valeo knock offs are easy to find and cheap, but it seems folks who can time ignition are a bit scarcer, particularly on this forum
http://www.euromotoelectrics.com/category-s/1614.htm
