This is a small company producing motorcycles but offers its engine as as standalone unit ready to run. How about one of these in your Mini's or whatever else you can think of (a light weight boy racer) ! Check them out https://motusmotorcycles.com/americanv4
Expensive...YES around $15000 for a crate engine and bikes are $36,000-40,000 U.S. I think , but listen to that American 180HP , pushrod , E.F.I. , V4 engine . Music to my ears . I believe it has a 9 inch bolt circle on the flywheel end to make adapting to almost any transmission easy . Wonder if they ever thought about running the Isle of Man TT ? It already holds the record for fastest production bike at the Utah Salt flats !