I'd say it's shot, and I don't know how much of it would need rebuilt. Rebuilding only the gears and synchros causing the problem is a bandaid fix on a used transmission, and would be a waste of time. I'd rebuild the whole thing, swap in used, or upgrade to the AX15. From what you described it's not a clutch problem.
Check inside the shift tower first, make sure everything is intact and engaged.