It's hard to say without seeing it. I always find these type of problems require a lot of time staring and fiddling with the machine, and trying different things, like pushing the shaft in, side loading it, manually operating everything every way possible, etc. Have you tried pushing the shaft into the case (like the clutch cover would do) while operating the lever? I would suspect a bent shift shaft before anything internal, but detent springs break, and clutches stick, so there are a few possibilities. Hope my rambling helps, let us know what you find.
1985 Tri-Z 250
1985 ATC250R