You can put an SR20DET a turbo 2.0 litre or a CA18DET 1.8 dual overhead Turbo both were available in Japan and AUS but none put out in the US you can buy both on eBay though knowing because my dream car was a silvia..im not sure yet but you might be able to put a 200sx CA18ET engine from a RWD 200sx that's what im working on finding out myself we have two junkers one with a bad frame one with a bad engine and we are tyring to make one runner.AnswerYou can put in damn near any 4 cylinder Nissan makes, although the transmission will not bolt up... meaning you will have to swap transmisions and yolks to, then order the correct u-joint. I know some of the motors that will swap with minimal customizing. Your has a KA24E(single over-head cam 2.4L), it can be swapped with the KA24DE(dual over-head cam 2.4L), the SR20DE/SR20DET(dual over-head cam 2.0L, DOHC 2.0L turbo), and I think a ZZ24(ohc 2.4L, 8 spark plugs). All of those will bolt up to your stock transmission and are a tremendous upgrade at that. AnswerThere are quite a few options for you.
Your car comes with a KA24E - a single cam, 2.4 liter overhead cam motor developing about 130 horsepower.
The most simple swap would be a KA24DE, the same motor as yours but with an extra cam, giving you an extra 15-20 (debatable) horsepower.
The next one up is a CA18DET. These came in the pignose S13's from Japan (1988-1990 I believe...) and some European versions of the S13. This is a 1.8 liter Dual overhead cam turbocharged engine through a Garrett T25 journal bearing turbo.
The next is an SR20DET. Two liters, dual overhead cams, T25 journal bearing turbo. Nothing too special, an S13 SR will net you 210, an S14 (with which you get a bigger T28 turbo with a ball bearing) will get you 235-240, and an S15 SR with which you get an even better turbo, and some other fun things will get you 250. Everything above an S13 SR will be more money.
If you are adventurous, there is the RB (or Race Bred) series of motors from the Japanese (and European, Australian) R-chassis Skyline. RB20 = 2.0 liter RB, T25 single turbo, etc. RB25DET, 2.5 liter straight six making close to 300 horsepower. The RB26DETT (or "Godzilla") is a true monster, an iron block with two turbos, 2.6 liters of displacement and a pretty good bite, will net you just over 320 horsepower. Or so. Most of these are relatively easy bolt-ups.
For the truly adventureous, there are hybrid swaps such as an LS1 from a Corvette, a VQ motor (Nissan, came in some G35's, etc...) and many, many others. If the basic geometry is applicable, it's been swapped into a 240. Someone on NicoClub has recently finished a V10 Viper motor...it's a monster.