When I dropped my motor last year, I wound up DESTROYING my front coupling ( and almost the bushing) and bending the shift rod that comes out of the bulkhead just to chisel the hopelessly frozen magnesium coupling off the shift rod ( UGLY!). It was a case of the different metals welding themselves to one another.
I had to buy a new coupling and bushings when I reinstalled
the motor ( used copious amounts of anti sieze on the end of the shaft) and that front shaft is still a little crooked( but it shifts fine).
If this is your problem, maybe there is a better way to remove a stuck alloy coupling from a steel shaft. Anyone have ideas on this?