There is another way to go that has not been discussed and trashed, that is a total conversion to a Vtwin of your choice turning a generator that powers an electric drive rear wheel. Eliminate the Centa, clutch, transmission, driveshaft, bevelbox, and rear drive belt and sprocket and put an electric motor on the rear wheel.
Smooth power and an engine that is readily available, like a MotoGuzzi, Harley, Honda, etc.
Don't forget you will need an HV battery and an inverter for the drive motor
Our diesel electric boats in the navy and the ferry systems don't have giant batteries for storing power, they just regulate the power to the electric motor. The first diesel electric boats in the navy might have been submarines and they needed batteries for running submerged, and I doubt the M3W will need that requirement, but in England one never knows :-)
The only battery necessary would be the engine starting batter which we already have in place.
As to electric efficiency, the efficient part is the engine running at a more constant rpm if you vary voltage with a big rheostat rather than varying engine rpm.
