The Ducati Cucciolo (Puppy) engine design was offered to Ducati Elettronica at the end of WW2. They took on manufacture and development of the PULLrod 50cc 4 sroke, and had a success on their hands. It grew to 65 later and gained a 2 speed gearbox.
There were many thousands made, both by Ducati and their licencees.