It won't be an easy conversion, because engine is longitudinal and engine bay is pretty small. So you need a very small engine, that can be bolted to the original gearbox. Maybe 1.0L Daihatsu Charade engine? Don't know if you'll find it in US.
I'll try to find some more info on engine swaps for this car. Lets hope I'll get back to you with some useful information.
Good luck with your project anyway
