Olaf,
I've attached the exploded view of the C gearbox taken from the two stroke parts manual. You can get any of the engine manuals on the Rotax owner's site: http://www.rotax-owner.com/en/suppor...engine-manuals though you might have to register to access them.
The oil seals are parts 10, 24 and 13. So far I've replaced 10 and 13 with non Rotax parts and have had no further leaks.