My thinking exactly. Trying to engineer a smart engine into an airplane makes as much sense as making it your life's work to convert gold into lead.
The Rotax 912ULS is a 1.35 liter, 140 lb. four banger and has a max continuous power rating of 95 hp at 5500 rpm.
I don't know what the smart's mitsubishi 1 liter weighs, but I'm liking the sound of a 95 hp german Rotax!
Alas, the downsides are obvious. Mating and fitting an airplane motor to my smart would probably be one heck of an engineering job, if not impossible or have unacceptable downsides. Likewise a bit of research leads me to believe that a fresh 912ULS would alone set you back around $17,000!
Maybe the Rotax V990 at up to 135 hp with 6 speed gearbox:
or the 804 at 85 hp:
Well we can dream, can't we?