Taking off the timing cover, presents a wrong gasket. This wrong gasket covered an oil channel between the engine case and oil feed pump. If this issue is also responsible for the engine breakdown, I don't know. Definitely it wasn't helpful.
Over the time I'm getting more convinced, that there are more issues with this bike.
With the help of my home-made tappet guide pulley I was able to pull the guides out of the engine case without any damages. I used a conventional pulley, in combination with an old valve spring cover. It turned out, that this worked perfect
The bushings of the connecting rods, as also the crank pin, shows visible deep scratches. Something definitely went wrong. I assume, that parts of the broken piston rings went into the oil feed circulation flow.
The troubles with the flywheel blows up my previous defined plan. Changing the bushings of the connection rods, without having the required tools and experience, I have to quit! To assemble the flywheel by myself, maybe yes, but truing the flywheel, no!
Finally I have to admit that I cannot perform all required repair jobs by myself. I have to search for an expert.
I cannot state, that over the time I become more and more confident. The project seems to turn into an adventure with an unpredictable end.