I found some info on the history, Fuch Hilfsmotor FM40(S), 2-Speed 38cc, pretty rare. Two versions were made, first one from 41-51, the second from 52-55.
This is the '53 model:
I found the Original 1953 Installation/Owners manual, neat to look through. It's Swedish, but Google does a nice job translating: BRUKSANVISNING FÖR FUCHS CYKELHJÄLPMOTOR FM 40 S.
Looking forward to pics, to help narrow down your model/year. gl.