I can see it for a race bike used on a closed track maybe, but for legal riding on the street, since a large majority of states have 50cc or under regulations, widespread use would scoff at many existing laws.
Actually, something like this or any of the HF engines IMO are far too large and heavy to be classified as suitable for a true motor assisted bicycle.
Mounted in-frame, they end up too wide and by default end any effective use of the pedals. Factoring the engine weight plus oil and fuel, not to mention whatever the gear reduction and final drive train weigh, begins to move beyond the idea of a helper motor and more into the realm of scooter or small motorcycle.