It's a 240sx so most likely a SR20DET though there's always the chance of a RB26DET. I could also imagine, if there was enough time and effort put in, a 1/2JZ-GTE or even Ford Windsor/Modular or LSx. Then there's always the chance of someone swapping in a 13B-REW or leaving the stock KA24DE, but if it's stock, there is no reason for this thread.
That should cover 99% of all 240SX swaps.
It's running cali plates though so I'm not sure if any of those choices would pass CARB approval besides the KA24. No idea though, never lived anywhere near California.