Hello Johannes,
thanks for your tip. I already contacted Mr.Treiber and he said that before anything is done (welding) at the frame I have to sign a paper that the street permission for use of the motorcycle in gemany is lost in this case, and that he is in no case responsible by law for the things he will do with the frame. This thing and the fact that I don't wanted a welded frame lent me to the decission to sell the frame and look to another one, which I did meanwhile with more or less success.
Hans.