Engine and fitting...
Assume that the Universal diesel is specific to the question because one is being offered....
If a wider choice is being considered, you might like the comment from a friend of mine that replaced his ailing A4 with a Yanmar a couple of decades ago in an E-27. I sent this thread along to him and his reply:
Loren: Seems to me I figured 10 HP was enough based on weight to be pushed around (not to mention size of pocket book to be pushed around). I just made a plywood mockup having the proper spacing for the four engine mounts relative to the output flange on the tranny. With that I could see just how to fit it in, what had to be done to the bed logs, accessibility, etc.
Not a whole new answer for you, but some more data to ponder.
I recall noticing that the single cylinder diesel looked rather small in the engine compartment that had held an A4 gas engine before!
Best,
Loren