I am having a problem with the term "fret".
I am guessing the box drops down over the rudder shaft after the cover is removed and mount to the hull unbolted. You migh want to put a hose clamp around the rudder shaft to keep the housing from going down to far.
You may have to rotate the wheel to get the gears to release.
I bet just replacing the key with a new one will take the play away. If not all then look for shims on both sides of the pinion gear. Since the gears are captured once the cover is in place I see no need for bigger set screws.