Heavy Fuel outboard?
A recent issue of AV week discussed a new engine being created down under for small UAV. 4 -120 HP or so.
Their concept is to run JP (kerosene) and light it off with a spark plug.
This gets away from the need for really heavy construction to compression ignite (diesel) and creates a light engine.
As power is required the engine is a 2 stroke , and they believe fuel injection with high enough pressure will have such small atomised dropplets that the will vaporise rapidly , and be subject to spark ignition.
Their hope is for better lower fuel burn as no fuel will be used to cool, and carried out the exhaust .
(Guess they never saw a Steyer Damler Puch split single from 60 years ago)
This research will probably be patented , so the Chinese can copy every concept and perhaps a heavy fuel dink outboard is in the works. With no mandated Ethanol the fuel could be good all season.
At 120HP most of our cruisers can get by with 1/5 to 1/2 power so perhaps they could be cruising boat sized.
Air cooled would require some space loss for ducting , but winterizing , or shallow sandy water would not be a problem.