Daunting tasks I can no longer put off . . . .

Having made the rudder, the next job was the tiller - but this would have to be shaped to clear the motor which I hadn't, at that point, bought. So what size and make of motor???
The most general rule of thumb seems to be 1 hp per 500 lbs of boat weight. My best estimate of weight with 2 adults and assorted kit on board is around 600 kg - say 1200 lbs - so that a 2½ hp motor should be ok. However, most motors this size don't have a reverse - you have to swing them around 180 to go in reverse so that the tell tale them sprays into the boat. For a reverse gear you seem to need at least a 4 hp motor, so this is what I opted for. Any more would seem superfluous since this length of hull is never going plane or go beyond about 5 knots; if I find myself facing a 5 knot current, no amount of hp will take me forward. 
As for make of motor, I compared all the technical stats. until my head hurt and I concluded that it actually doesn't matter all that much - so I bought a black one!
Now I was in a position to construct the outboard well.

The first thing to do was to make an accurate mockup of the outboard well in chipboard and then to mount the motor in it so as to gauge the minimum size of cutout in the hull. The plans allow for the motor to tilt up into the boat so that the cutout needs to be long enough to permit this. It would be possible to cut out a small circular hole to clear the prop. and then lift the motor straight upwards but it weighs 26 kg and then there's nowhere to stow it, so an elongated cutout seemed a beeter option. It looked like this:

a template is bonded in place which will serve as a base for hull inserts; lots of small holes can be seen here to guide the cutting out. The result is this showing the cutout and the resultant hull section that is to be used as a plug-in when the motor is removed:

The plug-in gets a lip bonded to it with seals on the underside which then fits back into the cutout and is held in place by button catches.

 An identical insert is then shaped but with a cutout and gasket fitted to go around the otboard leg. The gasket is 2 mm neoprene. On the underside of the hull, a 2nd gasket is fitted forward of and overlapping the 1st.

Then I could at last fit the motor and then I could shape the tiller.