The seal on the shift rod:#1, try lubing it as you pull/ push as you try going into gear.
You need to turn the prop as you shift, if not the dog/gears won't mesh and won't go into gear.
#2 replace the seal on the shift rod, make sure the surface is clean, corrosion builds up under the seal and causes pressure
on the shift rod.
#3 check for rust, corrosion in the gear housing.
#4 there's 2 nuts on the top of the shift rod(under the carb) they can be adjusted for length of travel. The air intake has 4 tiny screws, remove the intake and it's a lot easier to access the double nut.
Good luck and welcome to the site.