Well, there are so many options for engines that will fit into the 240sx. First there's the RB series motors, the RB25DET, the RB20DET, and the RB26DETT. They are motors that would be a job for more of an mechanically experienced person though. Then there are the SR motors. The most popular is the SR20DET which is the most commonly swapped motor among the 240sx enthusiasts. The RB motors offer anywhere from 205h.p. to around 275h.p. The SR motor is the easiest to swap, but offers signifficantely lower horsepower. I would probably recomend the SR for your swap. Now, to modifications that would be needed. There really aren't many modifications that are needed in a swap of one of these motors. Accept of course new motor mounts. And most sites sell you all of the components of the motor along with the engine and transmission so there isn't much, if anything that you'll have to buy to get it all working. I'll give you this sit that sells Nissan motor swaps and they have great prices, some of the cheapest I've ever seen.
http://jspecauto.com/catalog_c...d=111