Hi again
The new gear box will have a 3/16" output shaft under the aluminum prop hub. You will need to drill or file the drive cup to 3/16". If your prop shaft is like mine ,it's 4mm dia (5/32") and about 8 in long. Offshore Electric sells a Cen boat part cen-BJ14 prop shaft($8.99) that will easily fit your boat. It's a little too long but is easy to cut off to the legnth you need . The prop end is threaded 4mm so any 4mm threaded hub prop can be used. Offshore also sells a good selection of Graupner carbon fiber 4mm threaded props that would work great on your boat. Its the K series props you would want.
The prop shft also comes with a drive dog and 4mm to 3/16" adapter to allow the use of 3/16" hubed props too.
The advice is good about using a longer motor (550) like the 600 graupner motor. Even with the gear box you have a pretty good sized boat to move. You can save a bunch of weight using 2s lipos instead of a standard battery pack too. The idea is "to go faster add less weight".
All of this come from a lot of hair pulling a long time ago when I was new to rc boats.
n.h.schmidt