Finally the new dynamo arrived. It turned out to be a different model (not for this Pontiac) but using one part from the old dynamo it fits! On 24 March I installed the dynamo but couldn't get the car running. As it was late I left it as it was and continued fiddling around the next day. It turned out that I just had to put some fuel direct into the carburetor as everything dried up. After putting half a cup of fuel in the carburetor the car fired up immediately. Didn't expect that to happen after a month!
The dynamo is not the original one and, more important, it doesn't recharge the battery enough when I use lights and radio together. This means I have to be on the look-out for an original one, but as the car runs fine with the existing dynamo I'm not in a hurry for this.