Franken duck had a thread about this but as usual I can't find it.
There are 3 gear sets K75 3.2 - 1, K100RT 2.91 - 1 and K100RS 2.8 - 1. There are others available but not for an easy change.
Check if the drive shaft has 16 or 20 splines on the rear end of the shaft as most rear drive units are 20 spline so if yours is a 16 spline you will need to change the shaft depending which drive you get.
Other than the splines it is a simple job and would take about 1 1/2 hours at most. If you need to change the shaft then 3-4 hours.
See the thread in the lieberry about doing a lube on your splines and you will see what is involved.