Laurens,
I too would have to suspect the crankshaft oil seal at the output shaft from the engine assuming that the obvious things like the sump plug is in place and the external oil hoses are connected. I don't have access at the moment to my photos of the back of the engine when it was apart to change the compensator but I think that the oil seal is visible when you have it stripped down to fit the new compensator, so I suppose it is possible that it could have been damaged.
It is also possible that a leak from one of the oil hoses up at the tank could be pouring down into the compensator housing through the opening on its top side where the steering rack sits; any oil getting in that way would pour out of the bottom but the leak should be obvious above the housing in that case.
