Here's some news from me. I've sent the cylinder with piston, head and crank to Motorrad Racing Grün here in Germany. After inspecting the parts he told me to get the genuine piston from cmsnl, as he does not have an alternative that would fit. Luckily I waited with ordering the piston until he called me, because of wear he recommended me to go for 3rd oversize instead of 2nd. Good for me, the 3rd oversize with rings is cheaper than 2nd.  

While waiting for engine parts, I started to rebuild the front end. I thought the fork would be in decent condition, but turned out slightly bent. Also the springs are different in length and don't look straight anymore. I put new seals and dust caps on it anyway, as I don't really have a better idea right now.
Filled it with engine oil SAE 10W30 as said in the workshop manual. Well, I think it's okay for now, but honestly the fork doesn't feel great when compressing it by hand. This made me already think about looking for an upgrade, any ideas on that? Preferring models that are more spread in Europe, I would like to avoid buying big parts like this from the US right now haha.