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.