Before you pull apart the aftermarket starter, can you check to see it's spinning the same direction as the Rotax one?
I would be surprised if the aftermarket one isn't pretty much exactly the same, other than the possible direction of rotation. So it should have an oil seal, since the engines these starters go on have the same set up as the 912. That is, sprag clutch in the crankcase or gearbox, running in oil.
Looking forward to more news 🙂