Gary-
The wrench/socket size for the exhaust nuts is 1/2".. If the missing nut is on the rear cylinder, I can't remember if it is the upper or lower one but you have to use a 1/2" open end wrench that has been cut in two (meaning the wrench can only be about 2" long). With TD's, it is hard/impossible to get a socket on it. On the other three, I use 1/4" drive ratchet and long or shallow sockets with extensions as needed. Hope this helps. I carry a couple spare nuts after reading Hogbreaths problems last year.
If you don't see any black soot around the exhaust flange, i think the gasket/doughnut is ok.. The part # of the nut is 7593 ..
Bob