The clutch is now mounted, together with all the parts of the kickstarter, without any incidents. The water pump drive shaft was damaged, it was quite corroded and leaking because of the worn mechanical seal, which damaged the whole shaft afterwards.
Normally that little shaft with the bearing costs about 250 €, at this point I have to thank Harry, who gave me the shaft from his decommissioned RGV250 (also a living room project). Shaft, shaft sealing ring to the clutch and slide sealing ring to the water were exchanged. Unfortunately I did not manage to remove the old mechanical seal without destroying it.
Inserting Kickstarter Mechanics 
Inserting the outer clutch 
Clutch Parts 
Inserting the Clutch 
Old seal destroyed during removal 
Water pump drive shaft parts 
Installing a new sealing ring 
| Previous | 3848,18 € |
| Water Pump Seal | 21,62 € |
| Engine Shaft Seals | 43,47 € |
| Sum | 3913,27 € |