I did the same exact thing on my M103 - dropped the nut down in the crankshaft pulley area and spent 1.5 hours fishing it out with a magnet and piece of wire.

As far as the O ring goes, I found a piece of clear polyethylene tubing about 1/2" in diameter and sliced a 1/8" piece of it to make my own O ring. Then I tightened up the nut and just squashed the new ring against the old one. That was 2 years ago and the shock is still quiet.