In case anyone is wondering, this swap is very possible although with the expense, it may not be worth it to you.

Basically, you need the following:
- late M104 a/c bracket (A104 236 01 30)
- late spring loaded tensioner
- late tensioner pulley (104 200 10 70)
- late rear powersteering pump bracket
- and the big one...late a/c compressor and appropriately modifed hoses

I have everything but the compressor and hoses which I will most likely source used. If you don't want a/c, you can leave the compressor off and use a shorter belt.

So, to answer my own question, yes...the newer M104 spring loaded tensioner will bolt-on to an M103 engine with the above parts. Is it worth it? You'll have to decide that for yourself.