Fiat is making a 57 mpg car from a TwinAir engine. It will be a two cylinder with up to 110 hp for the turbo version, or as little as 65 hp. That might be more than you need given how small the car looks. Wired is speculating on whether this will be introduced in the US. I sincerely doubt it, though it's possible work on this engine may lead to introduction to other smaller, more fuel efficient engines from Fiat in the future.
GM is upping their game by letting Corvette buyers assemble their own engine (for a price of course). They're charging $5,800 for it which is pennies compared to the price of the Z06 or ZR1 engine. If I were a Corvette buyer (and had that kind of cash laying around) I'd definitely take them up on this. How neat for the engine enthusiast but non-expert.