I have a disco manual transmission for a 300tdi. I would be more than happy to swap for a 90 tdi transmission
I believe I also have a radiator and intercooler for a disco.
I haven't done a disco conversion (only defenders and ltwt), but you'll probably be looking at welding new engine mounts to the frame. pull the disco fuel pump, and new engine harness Once you've done it a couple times it starts getting easier
Before you put the engine in, do compression tests on the cylinders, have the injectors checked, pull the turbo and the injection pump and have them checked out. Also change the timing belt. Seems like going overboard, but unless you pulled the engine, you can't trust it. And believe me it's easier to do it on an engine stand prior to installation