Those units can usually be recovered with a bit of basic maint.
Cables, if seized, tight or leaking rust should be replaced. Take a pic of the shift and throttle assembly before removal, to be sure you put it back the same way. If incorrectly assembled, fwd can be rev. You have been warned
Then strip the cables and connections, and remove the unit from the boat. If its real bad, you may need to soak overnight in penetrating oil, nut cracker or similar, then try to press out the seized gear select disconnect knob. I've usually managed to get them out, but I did manage to break one