I've read of lots of swaps... Mike's looks clean and roomy, Jose's kit addresses the needs of some folks, and other people have swapped in drivetrains from an RX7, a Nissan (recent thread), Pinto (2L), Ford V8 (of course), Chevy V8 (father and son project I saw at the Indiana Invasion) and even electric. Apparently the recent Nissan is a really good fit, a Miata transplant is underway (I believe) by someone on the forum, and I spoke with someone considering a Honda S2000 swap, too. Ztech and duratech and Quad4 have been mentioned also, if not already accomplished.
All of them will require grafting engine wiring harnesses with the existing system, all require exhaust work to varying degrees, and many require bodywork adjustment underhood or in the trans tunnel. The key issue there is retaining the tubular braces (see Mike's photo above), preserving the structural integrity of the x-frame, and having enough room up front for the radiator. This past Invasion had (at least) two cars that reaped the benefits of pushing the radiator (non-stock) well forward of its original position. This allowed generous room ahead of the engine -- even more than a standard Alpine. (One of those was built using Jose's kit, I believe.)
Horsepower usually governs the rear end choices. A Miata or Mazda swap would likely remain compatible with the Alpine rear end. V8s, not so much. Of course, if you never punch it, the standard rear end is fine with a V8... but really, how are you not going to punch it?
Stock steering setup relies on some room behind the engine, so the choices there include Jose's kit, or the rack and pinion shown in Mike's photo.
Search this site, (and others) for info on what works, and what doesn't, when it comes to Alpine engine mods. And also consider what a car would be like if you put the same amount of money into the original drive train. While it won't spin the tires in all 4 gears, there are quite a few mods out there to improve the performance of the standard engine.
On my car, the philosophy is: changing things is fine, but it should be able to be returned to original by the next owner, if desired.
In my book, it's all good, if the car does what you want it to do. After all, it's your car!
Ken