[gtdump]

It's possible that one could find a Japanese kei car with the 3b21 engine, assuming there are some, and it might be a drop in replacement.
There's a long history of swapping different motors into cars going back to the earliest days; no reason one couldn't do so in the Smart if you found one that fits or are willing to do what it takes to make it fit. There are a couple of choices in the 1.0-1.5 liter range that could likely be made to fit.

If Bad Obsession Motorsports can put an AWD Toyota Celica drivetrain into an old Austin Mini without changing the outer dimensions of the Mini(*), then it's just a matter of of know-how, determination, money, and a little madness....plus a few brackets.

*)

 

[gtdump]

There have been a number of Smart car engine swaps, most go in directions I am uninterested in like the Hyabusa, Honda K-series, etc. Most of the time it seems the goal is jam far more oomph than is reasonable, fine for them but I'm more practical in my efforts. The Smart will already go faster than I need, so bigger isn't necessary as I need no help getting tickets. My focus would be something relatively compatible but more maintainable if I needed to, but even then it's going to be a while before the Mitsubishi is likely to become unmaintainable. The cost to put in something else is almost certain to exceed the cost of acquiring a replacement Mitsubishi; sure they aren't cheap, but they're not that expensive.

With advances in CNC machining and 3D printing, creating one-off parts could become even more reasonable than it is now. I've created a few parts here and there for my cars and some older farm and industrial machines; a small home foundry, some basic machining equipment, and time can produce some amazing results. Some people I know on other forums have recreated 4 barrel carbs, mainfolds, water pumps, and other various odds and ends, so it really is a matter of determination.

My second Smart came to me as a junker, the head warped when the serpentine belt failed. I replaced the engine and the old one is on the healing bench waiting for me to get to it. I rarely dispose of parts unless I can replace them cheaply and easily. I have a spare set of shift motors which need new brushes and bearings I hope to get back to some day as well.