It’s possible that you can try to do some adjustments from the outside, and it’s possible to check to see if there is lots of wear on the shifter parts
The photos are from a new MS2 inboard transmission
, but basically the same as your saildrive
#1- from the outside...you see the adjusting bolt with its shims, which is mounted in the threaded hole on the outside of the shifter mechanism, also see the shift shoe & spring which is mounted in the shifter on the inside. The spring end of the shift shoe pushes against the end of the adjusting bolt.
Photo#2- The shift shoe located in the shifter
#3- looking in the gear
you see the groove in the shift cone where the shift shoe rides, and allows the shoe to move the cone up or down to engage the cone clutch
The pressure of the shift shoe against the cone allows the shoe to break the “suction” of the cone and the gear
and pop the cone out of engagement when the engine is running. When the engine is not running there is no “suction” so it shifts easily. ( It’s not really suction, but it’s the force generated by the cone being mounted on helical splines on its shaft.)
As wear occurs on the cone and shift shoe the pressure between the shift shoe and the cone lessens and shifting becomes harder. What you can do is remove one shim at a time from under the bolt head
and test the shifting. Hopefully you will regain shifting before running out of shims. If wear is too great you may have to replace the shift shoe or the shift cone.
If you decide to remove the shift mechanism to have a look for wear, be sure to reinstall the shift shoe the same way it came out. It isn’t semetrical, one end of the shoe protrudes further than the other and that end wraps around the shift cone.