With the proper fabrication you can get just about anything into anything. Don’t expect to find a bunch of “put motor X into a '59 Metropolitan” kits out there, though, so expect to spend some time and/or money custom fabricating mounts.
Also keep in mind that whatever you put in there, you’re going to have to upgrade the whole driveline, especially the rear end - - the original motor on that thing was something like 50hp, so no matter what you put in there you’ll probably at least double the power. And follow Texases’ instructions as well, upgrading the brakes, suspension, and tires.
If I were doing this project I might be tempted to try and stuff an engine from an S2000 in there. Quadruple the horsepower out of a small 2L engine. 
(edit)
Oh, and do yourself a favor. Keep the original engine in a safe place. If you ever sell this thing, the purists will want to be able to put the motor back in.