The usual advice is to find a good (engined) but damaged D1, remove the drive-train and install in your Classic.
The donor will supply engine, plumbing, gearbox and Transfer plus prop shafts. Most importantly, engine mounts for the diesel.

Apparantly, the later 2.5's can be tweaked for more oomph...

Or, bite the bullet and go down the tD5 route, which makes them an entirely new and better car. But be prepared for lots of electrical fiddling, especially if you want to keep (or have) air suspension.