I am a happy man now. I received a tip from a member of the Swedish MB-club that there is a workshop that has been around for ages and which still occasionally does some work on these engines. They mostly work with trucks, but there is an old guy in the shop who knows the MB engines from the 60's. This shop happens to be just a five minutes drive from my office, and I had never heard of them before. I called them a few days ago and they would look around to see if they could help. The guy immediately knew what part I was looking for, which is pretty unusual with these cars, but was quite pessimistic. But they called today and told me that they had the part! When I picked it up the office lady was a bit apologetic - "it's a small part but I am afrait it is quite expensive" and then went on to tell me that they wanted the equivalent of $15! I would gladly have paid five or ten times that to get the car back on the road.
There was a nice W111 280SE Coupé in the shop and one or two /8's. This seems to be the place to go to if I need help with my car. They do transmissions, injections and other driveline stuff so it is very good to know.