Those crazy Germans again? For a bet ( a bottle of beer ) this blacksmith built a working 3 ltr single cylinder engine for a bike without buying any new parts. Started with a piston and conrod from a tank,as he would, welded up a crankcase out of sheet metal, Passat cam shaft and Golf distributer. Idles at 150 refs and tops out at 800 refs, you can actually count them!
Any more homemade engines out there? How big? Do they work in a bike?
https://www.motorrad-bild.de/ta_exoten/3_liter_bike.htm