So, now is the carburetor flange between cylinder and finally finished. The original part, which I have not, unfortunately, was probably once made of cast aluminum. I built the new flange of a piece of aluminum tube and a 15 mm aluminum plate. The raw material I had previously bought on eBay. plate and pipe are screwed together with a thread 33x1.
Thus, the screw can not solve everything later, I'll pour in a screw-locking compound to the threads. What product do especially well? I guess the parts are later in operation about 100 to 150 degrees hot. Am grateful for tips. 
Addendum: on this old picture can be seen in about how the times have Originalflansch must have looked. 
