Are you sure that the play is in the steering
box itself or is it possible that the U-joints are wore out. Either way you are going to have to take it apart from the U-joints to get it out, it doesn't look like too much trouble. You may be able to have that steering box rebuilt, find another one like it or find one similar that would work. Maybe when you get it out, there may be a name and part number on it. Greasing the grease fittings on the gear
box and U-joints is pretty important. If you get a new one, I would still keep the old one on the boat for a spare, you never know. Good luck.