Haha, I was pretty much kidding about the AWD, but you'd have to admit it would probably be fun
As for the Mazda motors, I wasn't aware they were less desirable (I've never really looked at detailed stats for them other than some of the horsepower figures though). I've never seen any Kei in person other than sporadic trucks and vans around here in Nova Scotia. Very few people import the keis from Japan here aside from the trucks and most JDM imports here are things like newer Skylines, etc. I'm aware the Carol is mighty small!
Would the Yaris motor you mentioned fit in terms of measurements? It seems to be a good choice otherwise, they're certainly plentiful and 108hp in a Carol certainly wouldn't make it a slouch...
What about the 1.5L L15A w/117hp from a Honda Fit? I would think it would be about the same size, plus a tad more power. You've probably already considered it though.