No new oil stains were observed after the first cleaning, very promising. So the Rally was taken outside, and started as usual on the first kick.
Now, this bike is completely different from my SS; The choke has to be used for a longer period. On the SS, I use the choke for start, and then afterwards, I can run without choke. Here, the choke has to stay out for several minutes, and when I turn off the choke, the scooter will very soon die. Might need to something with the carburettor - later.
But back to the test drive: The scooter SMOKES a great deal on full throttle. An hour after testdrive I took these pictures:
The stain is petrol.
...and from the looks of it is comes from either between the cylinder/cylinder head or from the cylinder/exhaust port.
If I'm lucky, renewing the cylinder gaskets should cure the problem. Seems I don't have to split the casings afterall!