Straight swap is easiest, although even then you need to get the year reasonably close as there are differences in the loom and the ECUs. Usually people get them out of scrapped Range Rovers or Vauxhall (Opel) Omegas. Same engine was also in the BMW, of course. Unless the FIP ends 994 you may have to swap that over and if that is the source of you problem then you're back at square one. Where you find a genuine low mileage example over 20 years since production stopped is a mystery. Turner Engineering or similar might have a rebuilt one.
@stu might know where the race enthusiasts source theirs.
M57 can be made to fit but I have never done it. RangeRovers.net might have more on engine swaps. I have a vague memory of a kiwi doing it.