This site seems a good resource for comparing bike MPGs.
Of course, the figures are for engines pulling bikes and are often rider logged, so not any kind of 'bible', still...
Motorcycle Fuel Economy Guide
Using a bike with a shaft drive seems a good option.
Boxers should also have a lower CoG if they can be made to fit!
I realise that bike engines are usually skewed towards performance rather than economy, I just think the getting hold of a modern efficient & light, small-displacement car engine (in the UK) is going to be a greater battle.