Hi
Not a straight forward conversion. The gearboxes will have to be swapped as well as the engines due to the completely different shift points and ratios.
Getting the BCM to accept the new engine may be tricky, to my knowledge there wasn't an diesel HSE so how compatible they are I don't know. All the engine loom, ECU and sensors will need swapped, the whole fuel system as well.
Not impossible but certainly a lot of work! Probably easier and cheaper fixing the 4.6 and gassing it. Depends what is wrong with the engine, slipped liners can be fixed and the whole lot put back together better than new
If you need it, I have a chap who comes highly recomended. He built my engine and I have seen 2 others of his, they are brilliant! He is doing a P38 V8 for a guy on here at the end of July.
HTH