Advice please

My 4.6 engine on a late (2001) p38 has come the end of it's natural life.
The rest of the car is fine and I would like to keep it. So regarding the engine what are my choices - a part from a straight swap - could I go down the diesel route. Are the any other blocks I can drop in which would be worthwhile. Any thoughts would be appreciated.

2.25 petrol will drop in nicely.

The M51 diesel will go in no problem, have to do some changes in the BECM, maybe some probs matching up the 4.6 auto box the the diesel, certainly the bell housing and autobox ECU would need changing.
All sorts of other motors possible from Transit diesel upwards, all needing adaptors to mate with the autobox and electronic jiggery pokery to get it all working.

The cost of doing a petrol to diesel conversion would be expensive both time and cost wise. For starters the entire loom would have to be replaced together with the engine ecu, and the Becm reprogrammed. The diesel is "Fly by wire" whereas the petrol has straightforward throttle cable operation. Then we come to the gearbox/bell housing match up..or lack of it...hell of a lot of work and hassle, not to mention the cost !!!

I would stick to "Like for like" petrol replacement and possibly going for a LPG conversion if you wish to try and run a P38 on the cheap.

The auto gearbox fitted to the 4.6 petrol is far superior to the box fitted to the diesel which is "Borderline" regarding torque limitations. There is a general concensus on here that the auto box fitted to diesels is prone to premature failure when a diesel is "Chipped" as the additional torque achieved overstresses the box in time causing failure.