Brilliant question!
A ~45cc air cooled single rotor Wankel, with easily-accessed manual clutch and 3-speed manual transmission. Side-housing inlet and exhaust ports with conservative overlap and duration for smooth idle and best fuel economy. Carburetor with separate idle mixture adjustment. Easily-accessed magneto with separate electronic trigger and spark coil. Ultra-quiet exhaust AND inlet silencers. 15 or 20,000 RPM redline. ~5 horsepower or maybe much more
An available option for oil injection pump with separate oil reservoir.
All in one compact, sweet-running, lightweight, narrow unit that results in a clean, tidy installation.
Made in Canada, for 350 bucks
If I were one of you very fortunate few with more emotional energy, I'd be busy right now developing it myself. I have the ability but sadly, not the motivation. It would be the ultimate bike engine.
Ian