1. Refitting the case is optional, but if you use the bare board, either to save weight or make siting the unit easier, it is a good idea to put a wrap of tape around it to both insulate it and provide a strain relief for the wire lead.  The motor can also be separated from, and rewired to the board.  Already like that on some (older) servos.

2. Finding the right size wire sleeving for the drive can be tricky,  the motor shaft is very short, and sleeving tends to push itself off over time, usually just as the boat goes out of reach.  My preference is the flint spring from a disposable lighter.  I recently got 5 for £1, picked the inevitable one that didn't work and dismembered the flint carrier.  Downside is that not all designs have the same diameter spring, most have tapered ends, which can be persuaded on, and GRIP.  Some are not tapered and cannot be used.  There have been questions about the spring unwinding.  Bearing in mind the power levels involved, nothing to worry about.  The best possible arrangement would be something that fitted over, and gripped, the cog, that way there would be a positive drive and no need to remove the cog.

3.  You use the pot as a center off setting control, but on a servo the deadband is very narrow, so fully off is not always easy to hit reliably.  One the other hand, if you just intend to park the boat, why bother with a motor?  One the other other hand (3 so far) there are robotics sites out there where circuit board mods are detailed to expand the dead band, but with my fingers and eyesight I have never bothered.

I start with the bearings - normally a short length of the plastic tube from a Q-tip or cotton bud.  I then find some brass rod that is a nice sliding fit through the tube.  That will form the basis of the shaft.  Next find a tube that is a reasonable fit over the Q-tip tube, thats the shaft outer.  The shaft needs to be lightly greased, the bearings part inserted in the tube, the shaft pushed through to align everything, and epoxy applied to the exposed part of the bearings before pushing them home.

Using the kit props involves CAREFULLY drilling and reaming a hole for the shaft and possibly some sanding of the blades.  Normally I epoxy this to the shaft.  I normally cut a sliver off the end of the tube and use it as a thrust washer/spacer.

It is also quite possible, with very basic tools and abilities to mark out and cut a prop from thin brass sheet, and soft solder it onto the shaft.

The master of this sort of thing, albeit at a larger scale, was the late great Towboat Joe, sadly no longer with us, but his site, and the information on it, has been rescued and is at  http://towboatjoe.tripod.com/

Similar procedure when making a working rudder.