I just had to buy one today 1.17 at local HD dealer. Never did find the original one, it fell out of my socket, and went who knows where. It does not appear to be anywhere on the bike, after a through search with mirrors, flashlights and magnets. Final torque is 120 Inch pounds.
Buy the flange nut and another 5/16-24 nut so you can double nut the header bolts so that doesn't happen again. Also get some anti-seize to put on the studs.