It's probably due to tax, insurance and MoT issues rather than it being the engineering.Why? doesn't look anything wrong with that, seems a nice engineered solution, there's a long history of engines being put on bikes.
Still not allowed in the UKI wonder what the rule would be for a petrol-electric bike ? Electric bike with a battery is perfectly ok, so what about a little generator set that keeps the battery topped up as you ride
A motorbike with no number plates etc., naughty.
The rear tyre doesn’t look much better than the brakes.The rear brake on that Frankenbike seems pretty effective. What's the problem?