what engines are also compatable for a 1991 nissan 240 sx....... ( stock engine is a (KA24DE ) ?

2.4 is the only option for that year. call a used parts yard and see what other 2.4 motors have the same basic long block.
Roy

If you are thinking of a transplant, the RB20 can be fitted in but you would need a complete transplant including the wiring etc.

thanks for the response so soon!... anyway i have heard that there are three different engines that will fit ( didnt here anything about wiring etc. ) but the sdr(?)is suposedly the best as far as performance ($1200) ? Was not trying for new but just looking at options, since im spending the cash

If you want something compatible, you need to get the exact match. Changing to a different engine means you need to convert the wireharness, engine computer and many other things.