I would like to
amend drinkypoo's
excellent write up with one
correction and a couple other facts.
The KA24E had 12 valves total, or 3 per cylinder, not 12 valves per cylinder.
The 1989-1993 240sx fastback as it is found in the US is identical superficially to the Japanese 180sx that was produced with the CA18DET engine for 89-90 and the SR20DET engine for 91+ (I believe production stopped in 98).
The 1989-1993 240sx coupe is unique to the US (and other countries that receive the same USDM model). As it has the 180sx front end but a s13 Silvia tail. So in effect it is already a one-via.
The convertible model, which I believe was only--and the only model--offered for 1994, is also unique for the same reason as above.
In 95-98 the 240sx switched from half-unique/half-borrowed styling and adopted completely the styling of the S14 Silvia, although in 98 the front changed a bit more to S15 styling.
As for modifications, a popular thing to do is swap the KA24 for a CA18DET, SR20DET, or rarely an RB series engine from the Nissan Skyline.