Originally Posted by Poolside
The OE master bore diameter is 16mm and the slave is 24mm. Based on the 'engagement range' of the OE clutch I'd guess that a 10% smaller master bore diameter still might work. Say 14.5mm as a minimum diameter.
But since Magura makes the OE BMW master, you might find a 16mm Magura part. If so, on the old and new clutch levers check the centerline distance between the clutch lever pin and the piston pin. For the same bore diameter, that dimension determines lever throw.
I'm more of a bolt it on, bleed it and see if it works kind of guy.
Clutch master is in stock, I have a couple of different ones I can try. Probably get to it after the new springs go in the forks and front end is complete.