Try to get some pics for you. Have been using either 2 x Tamiya R370 or 2 x R180 motors for destroyers, and 1 R180 for the missile boat. All running on 2mm shaft.
Didi get rid of the original gear box as it's noisy as you've rightly put it, and it breaks very easily.
paulsiu: do you have a build log for these ships in the forum? Would love to see what kind of motors and RC gear you have used to convert. Most of these kits have gearbox transmission to utilize one motor/two propellers. Problem is they are noisy and I do not want to use them.