^this. Check the state of your clutch cable and whether the stop plate under the gear lever is properly adjusted before ripping the gearbox out.
Clutch cables are cheap and easy to replace so I would do that as its probably worth doing anyway and it eliminates one of the most likely causes easily
Definitely go for all of the simple obvious causes before stripping everything down. Clutch cable , gearlever stop plate and shift coupler on gearbox nose as these are all externally accessible and just involve getting a bit dirty.
