Remember with the change to a diesel, you need to completely redo all the feed lines, pump, tank, filter, etc.
You could probably find a 4cyl from a 720 and make it work (carbed), but you would have to take the tranny and t-case for it to work. If you grabbed one out of a Hardbody (injected), you would only need engine and tranny since they use the same t-case, but also need the ECU since it is different than your V-6 one.
If you can't even do a timing belt right, do you trust yourself on swapping to a completely different drivetrain?