After checking, double checking and checking again, I trusted my wiring harness enough to connect the batteries and test the starter. Worked fine. I also had sparks at all three spark plugs.
But now, before I can run the engine for the first time (I’m slowly getting nervous) there was a small issue with the choke on the carbs I had to fix. Usually the choke got to lock in three positions. Mine didn’t. For some reason on the fist carb (the one closest to the choke lever) the small steel ball and the spring was missing. Got really no idea why. Luckily there is one in the third carb, that had no real function. Moving this thing over to the fist carb does not sound so good to me. First of all I got no idea, how to disassemble it. And second, to get there I need to touch the diaphragm, and I don’t like to take the risk of damaging it.
The easiest solution I found was to drill three small holes in the rod. That way the locking mechanism is working with the third instead of the first carb. The depressions came out a little bigger then the original ones, but that does not any harm to the function. When the engine is running and the electric is working, I will have taken a big step towards the my goal to have this bike on the road.