The original Bosch starter is no longer available and it has not been for quite a number of years. While it may be possible to rebuild the old Bosch starter, replacing it is worth considering because you will wind up with an improved part that offers better performance. BMW switched from Bosch to Valeo starters in the mid 80's and still use them on the new bikes today, so that is a testament to the quality of Valeo starters.
This part will fit all /5 models and /6 models up to 08/74 production. Equivalent to BMW part # 12 41 1 357 500 ( 12411357500 )
If your bike has a production date of 09/74 or later, you will need the 9 tooth starter, part number 1241670
IMPORTANT NOTE: for 1974 AND 1975 model R60/6, R75/6, R90/6, and R90S