It's possibly a head gasket or it very well could be corrosion. Either way der cylinder kopf most likely must come off. Don't just slam a head gasket on and slap it back together. Take the head to the machine shop and have them inspect it. This happened to mine and my machinist welded it up and did a valve job with new guides. New guides are really important on these engines.

I don't know how many people on this side of the Atlantic caught your profanity, but I lived in Germany for a couple of years so you made me feel at home.

Good luck with it,