I couldn't retrieve the clip with a stick with magnet. I think it was stuck to the left side between the wall and the flywheel. Moved and wriggled the bike, but no luck. I took the engine out, put it upside down, moved it a bit, and the clip fell out.
Actually it wasn't much work since I didn't remove alternator, clutch and primary drive. In 1,5 hours the engine was out, and a little hydraulic platform was handy to move it out of the frame.
A nice lesson to not forget

And I did immediately fill the crankcase cavity with a big rag
