A goal like that will likely a newer 125cc or even 110cc Honda engine, and just about all the tricks you could possibly find on here.
Low weight, highly streamlined.
Evomoto is getting 1L/100km - and he's on here BTW. ;-) - but that's on a 2 wheeled recumbent motorcycle.
Details: Honda - Innova - evomoto 125cc - Spritmonitor.de