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.

Fork tubes can be straightened if not too badly bent. If you have access to a press you can do it yourself. If not, there should be a shop locally that can do it for you. You will still have to dis-assemble them either way, to do the job.

Great news!

Here in the US, most 10w30 oil is "energy conserving" by law.  

We need to be careful to use oil rated for a motorcycle wet clutch as the slippery additives in car motor oil can sometimes make the clutch slip.

Alright, today I reassembled the front end with replacing the steering head bearing and I have a strong feeling that somethings terribly wrong here. When trying to compress the fork, it feels incredibly stiff, hardly made it to dip in. Could be the oil a problem? There might be too much in it, although I filled it with 134ml 10W30 just like the service manual said. Also I am questioning the oil choice, considering to try a modern fork oil as an alternative. I am just getting into the enduro / motocross world, but compared to my RD350LC, R5F and the DT175MX the fork just feels way too stiff and I don't think it's supposed to be like that. Best thing I can imagine is finding a fork from a different bike that might be worth an upgrade. Any ideas on that? Thanks!

When I was was Young and Dumb , I changed the fork oil on my DS7 road bike .
It then was very harsh and didn't dive much at all under braking .
Turns out I hadn't sufficiently drained the old oil out before adding the new fluids .
There should be a measurement for oil level in the fork , taken at full compression ,
with springs removed , typically 100 - 150 mm .
I don't have that measurement myself , hopefully someone else may have that figure .
I'm not sure whether that was included in service manuals in that era , but certainly
was later in the 70's . Also confirm levels match in each leg .

Bent forks?

Thank you for your replies!
I can exclude not having fully drained out the old oil from the forks, as I disassembled everything and gave the outer tubes a good clean out, also removing dirt from the bottom.
Also I don't think that slightly bent forks would cause it to be THAT stiff, I can compare to the fork of my DT175MX. When I got it, the inner tubes were terribly bent like bananas, way more than the ones from the MX, and the fork still seemed to dip properly.
I think next thing I'd try is to turn the inner tubes a little and pump out the oil gradually and checking if this gives any improvement. After that I would like to try a modern fork oil instead of the engine oil. When getting the air out while reassembling I could already see how the oil foamed up.
Also: the fork was as stiff as now before disassembling already, not meaning it's how it's supposed to be.

Depends greatly on where the fork tubes are bent.  Sometimes they bend under the triple clamps above the area of travel.  If that happens, they won't bind up. 

If they get even a slight tweak in the travel area, they will bind up.  I've seen a stiff set hit a good bump and get stuck collapsed.  

Solved it! I drained out the 10W30 completely and refilled the tubes with the same amount of 10W fork oil. Also found a position where the bent inner tube didn't make the fork being tensed up in the triple clamps.
Got a way better feeling when dipping the front end now, let's say it makes sense. 
Next thing will be giving the swing arm and the rear shocks and damper housings a paint job, going along with the throttle caps, foot brake pedal etc.