Just got off the phone with the LBS. Exact replacements of crank-set or rings aren't available. I can swap out from octa-link BB to square-taper BB, and get a similar crank-set, but then I'd spend about as much (or more) as I would on an FC-M590 (which I think I already have, boxed-up).
Right now I'm leaning towards...
- Have the LBS remove the old crank-set and "face" the BB-shell.
- Order an XT M770 11-34 cassette, for less than the cost of an HG80.
- Install the crank-set I've got boxed up, after I double-check it.
Going this way, my "out of pocket" cost is about $70 for the cassette and $40 for the LBS bench-fee.