Re: Replacment shifter
Sounds like possibly the shift cable if the throttle moves
and it is stiff. There is a tension adjustment underneath that control. I believe it is an allen screw. Tighter is, uh, well, tighter

Anyway, you might want to follow the cables to where they go to the engine and then the drive. Look for kinks etc. There is nothing wrong with that "style" control though. I like it better than my current one, which is also a Merc . . .