I have an old Troy-Bilt Horse model serial number 677311 from 1983 that I have had since new. I got her running perfect and was using for over an hour. and the wheel drive stopped working. Pulley is turning but feels as if it is free wheeling not attached to a drive shaft . My Manual that came with the Tiller states " shifting pin at the end of the Eccentric shaft may be broken. any ideas on if that is the problem, are the parts available and is it a difficult task I am pretty good but no transmission expert.

Thanks