Latest news:
Yesterday I took the cylinder head off. I didnt find any streaks there but I found some water pearls on the top of the piston. Then assuming that the smoke was because burning coolant I put the bike together again, and of course used new O-rings between cylinder and head. (The grease trick did the job perfectly. Impossible without it I think..). Now hoping that the new O-rings would solve the problem and that there wasnt any cracks in cylinder/head.
Well... got it together and started at second kick. And smoke was again filling the air around me and anything else..
Fearing te worst case that the cylinder was cracked I now emptyed the coolant and started again without coolant. Luckily (?) it still smoked heavy, so the conclusion now is that the smoke comes from burning gear oil.
So... back to the beginning. Starting dissassembly for the third time after work today...
When - if ever - this is fixed I will grant myself a long day in the woods riding my bike and then celebrate...
-Jan