If you are lucky the problem might be with the gear selector mech, rather than the gearbox. If the washer works loose on the end of the selector drum, the selector arm can occasionally jam in the gap rather than riding on the cam. I once had this happen on an ETZ and struggled to get above 1st or 2nd – I forget which, it was a long time ago.
Obviously the fault could be in the gearbox but checking this only involves removing the clutch and primary drive gear, and can be done with the engine in the frame.
Warning: if you remove the cam for inspection take careful note of which way round it goes!