None of them are legal. If it isn't a dot approved engine, its not legal. Period. And you are screwed one step further, cause the letter of the law says you have to put an engine from your model year and newer in, which means that even if you could find some north american nissan engine that would fit.... the only one that is actually legal really is the 350z engine.... which nobody is doing.
Drop in, the sr20 and the ca18 are both the same mounts, rb 20, rb25det, dett, rb26det, dett, vg30e, vg30de, vg30et, vg30dett are all engines that have been spooned into various s chassis cars... but if say a good sr or ca swap costs you 6-9k, plan on an extra 4 grand for any of the other engines, and maybe more. RB's are expensive for a dett, and have to be swapped to single turbo setups, cause the twin turbo can't clear the steering rack. The VG has been donei n the states a few times in the s13, but isn't worth the weight most of the time, just an exclusivity thing. Honestly, you wanna be different, keep the engine you have, and build up a torque monster turbo for it...
the good: There are 30 of these engines running around now that are cheap so if you blow, wh00p, 500$ for a new engine... and this is the closest to a "legal" app that you will get
the bad: its not an sr... etc etc... I have done some math with the prices I get parts for and stuff that I have found, and I could probably get a killer 2.4 liter turbo setup going for around 8-12grand... so right around the SR neighborhood.... ok, a modded sr, but nonetheless... and you WOULD be the only guy around who went this way!....