Hi I have a BSA boat that must have been made at the same place as yours. The gear boxes are weak and easy to break. A Master airscrew gearbox will easily fit . The 2.5 to 1 ratio is perfect. They are fairly cheap too. Either the output shaft has to be reduced or the plastic drive cup slightly drilled out and you are ready. As a added benifit you get ball bearings.
n.h.schmidt