Not to be a poo, but turbo on a gasser is a bad omen for good bsfc. And kei cars are all but non-existent here, I think someone had a honda beat and was getting maybe 40mpg.
Maybe you could sleeve a honda or metro 3cyl? Keep the longer stroke but reduce the bore. If Harley engines weren't so stupid pricey I would entertain one of those over a 10krpm motorbike engine for use in a car (maybe a harley clone) for efficiency.
The harleys were the first ones to toodle their way into the 100mpg range in the vetter challenge, and the 400+mpg bike had 1 less cylinder than that and was well optimized for size and long stroke.
I assume the goal is better efficiency.