There is some sort of a tension knob on the shift box. Although i cant remember if it was only for the throttle stiffness. Would be well worth taking that box apart again and making sure everything moves with clearance the way it should. I believe in the manual it says the cable should be crimped with little to none sticking out the end. just visible basically.
Have you also tried adjusting the length of the travel in the cable? at the back where the shift cable clicks onto the motor mount there should be a black plastic piece on threads. Try spinning it.
So put your shifter in forward gear. then at the back of the motor twist that plastic piece until you get the desired result.
the worst case scenario would be worn out gears in the clutch dog. you would have to tear apart the lower end to confirm this. if this is confirmed you have two options.
Find a parts motor with a blown head for cheap or, as some people have done, switch the reverse and forward gears as reverse is likely in much better shape.
Many people have done this since no one seems to mind loosing reverse (or simply holding the shifter when in reverse as is your case)
