Lifan 125 engine stuck in neutral

Have about 100 miles on a Lifan (Z-Star) engine in a 73 SL-70. While out on a road trip was down shifting, coming to a stop. Bike went into neutral and will not shift out, up nor down.

Big Rob and I switched the engine out today, thankfully I have lots of good help, but has anyone experienced this problem

I suspect that the shifting arm in the transmission has vibrated loose, and am prepared to pull the cases apart, with Rob`s help of course.