Ducati Bevel-gear L-twins
Source pics: The red bike is from tuner Alfred Bajohr. Pic taken from an article in Motorrad. nr. 4/1979 “Das Mass ist voll”.
The other pics are from another article in Motorrad:
Rosshuber, K., Der richtige Dreh: Ducati-Königswellenmotor: Technikreport, Motorrad (1987) nr. 7, blz. 58-69.
picture of the cylinder head on foot board
[Classification = Group 6 A]