Personally I'd go CA18DET. Bit older and perhaps not quite as much potential as the SR20DET, but a lot cheaper and should slot right in very easily, fitting to your standard engine mounts, transmission, etc. Can you get parts easily from the Bamboo in Trinidad? I'm sure someone there would have a CA18DET 180SX or Silvia front cut.
By the way I am almost certain the CA18i is throttle body fuel injected, not carburetted. Feel free to correct me if I am mistaken. This would make the CA18DET swap even easier as you would not have to change your fuel system (aside from possibly fitting a higher-capacity fuel pump - but it might be the same pump as any other R32 in which case you wouldn't need to).
It has to do with the combustion chamber shape, with a two-valve engine it's a bit tricky to get a spark plug right in the centre of the combustion chamber, so putting one plug on either side of the chamber gives you more complete combustion.