The GA16i was a sort of transitional motor. 86-88 had the E16, which produces about 70 hp. The GA16i produces about 90 hp. From what I've gathered, the most bang for your buck motor swap would be a CA18DE, which comes from the 88 Pulsar SE's.
However, if I were you, I would just see if I could get another GA16i. I think in japan they are numbered a little differently. Something like a GA15i, but still essentially the same engine. 1.5L as compared to 1.6L.
In 1991 the sentra was completely remodeled, and the chassis was redesignated from B12 to B13. A new engine was also added to the mix - the GA16DE with dual overhead cams, and multi port fuel injection. www.sentra.net
has tons of info on this engine.
If you're a high roller, there's really no substitute for an SR20DE or the turbo version, an SR20DET. These engines came in the 91-94 Sentra SE-R, and have much more aftermarket support. They are expensive, and rightly so. However, it takes quite a bit of doing to make them fit in a B12, as they were designed for the B13.
The way I see it, the CA18DE is to the E16 as the SR20DE is to the GA16DE. The GA16i fits somewhere in the middle there, but shares motor mounts with the E and CA-series motors. The Pulsar and Sentra share chassis designs, and the pulsar even came with GA16i's in 89-90. So the way I see it, the CA18DE is the most convenient solution to boosting the power of a B12. Sure, it doesn't have as much aftermarket support as the SR20, but it's a hell of a lot less time and money that could be spent actually driving. Not to mention that the GA16i is already adequate for most people, so a CA would just be teriffic. Of course any motor swap is going to be a pain in the ass, since there's all that other stuff that needs to be swapped along with the engine. That includes the wiring harness, ECU, sometimes transmission, axles, and in some cases, the fuel pump as well.
I like the GA16i - it does fairly well in the sentra. It's reliable, and a gas-saver to boot. I'm sure you could find one in decent shape around somewhere. I hope this was helpful, and good luck!