Things are moving forward
I carefully removed the transmission last week:

I brought it to the workshop already, which will do the rebuild. I hope they will contact me next week so I can take a look inside whats the problem. I'll do some pictures if I get the chance.
Beside this "transmission mission"

other things are going on too:
I took the power steering pump out, cleaned and painted it, and did a new (liquid) seal between the pump und the reservoir - another big leakage eliminated
I also replaced all water hoses - including removing all the water flanges, sanding and painting them, new seals, new screws tec. I also took the thermostat out and literally coocked it, to check if it is still working as it should

Forgot to take more pictures...:

A few days ago I ordered the big gasket set for this engine, and also the engine paint in the original color.
I' will remove the engine soon, "beautiful engine" mission incoming
I'm still not sure about the compression I measured -
is 110psi a good compression for this engine?
The answer to this question would have a major impact if I do the small engine job (all gaskets, cleaning, new painting) or turn the big wheel (cyl honing, new piston rings, regrind valves etc.)... Any hints are welcome!