Multiple possibilities - shift shaft bent or otherwise damaged and binding where it passes through the engine case, damaged shift arm on the inside end of the external shaft shaft where it connects to the internal shift shaft going to the front of the engine, broken centering spring on the forward end of the internal shift shaft shift mechanism, a combination of problems, something entirely off the wall like a bent or loose centering spring anchor pin or something else.
Best to take it to an experienced mechanic for an examination and based on his diagnosis possible engine removal for repair.
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