Hi Per,
The motor is very primitive, with its bronze cases, exposed timing gears and the coarse fins on the barrel. These are all hallmarks of a home-made motor, and I'd be pretty certain that's what we are looking at. No production engine would have these features, particularly the lack of enclosed timing gears, even pre-1900.
Magazines like "The English Mechanic" gave serialised instructions on how to build your own motor, and many enthusiasts gave it a try.
The motor looks quite large, so it could have been built for a car rather than a bike.
Is there a story about where the motor came from?
Greetings from Australia,
Leon