Non-printed parts needed are bearings, shaft, valve, timing cam and belt, switch, ignition circuit etc.
All parts are metric, but if you can find a close fitting imperial parts they should work too.
Modular design allows for 1, 2, 4 cylinder arrangements.
For water cooled parts, print them with gyroid infill and cut the end of the pertruding structure.
The author marked this model as their own original creation.