I've been dreaming of Elio striking a deal with Ford for the 1.0 EcoBoost. Not only would the car get crazy good fuel economy, but it would be a rocket. That little weight with that much horsepower would be way fun. I'd even pay a couple extra thousand for that.
I love the 1.0 3 popper as well as the other brands coming out (Mini has a 3 cyl. turbo now, etc.). I can't wait til these engines start hitting the junkyards so I can use them in projects. I've heard that some of them have been tuned to 200+ HP. All sorts of awesome in a lightweight platform.
If you're looking for a tiny, lightweight motor for some sort of homebuilt project, you could look into using a motorbike engine. Many of those in the 900-1000cc range can be used in heavier vehicles and are already up in the 150+ horsepower range. The transmission will be a little more work, but I've read articles about a guy with a Miata with a 1.3 liter Suzuki motorbike engine mated to a traditional transmission in a Miata, all in the name of minimum weight.