Hello Antony
You now have two threads running but I thought it would be easier to reply on this one to avoid confusion and because this is where you have the photos.
The hull closely resembles a Dragon class which if you Google you will find loads of information about it. It is a classic international design and still raced competitively. Even the small cabin looks very much like the cuddy of the real thing so you could very easily retain it. If you do adopt the Dragon you can use it to scale everything else.
Are you certain that the mast step, cuddy and breakwater are not on the centreline? If the planks are drawn incorrectly it could be an optical illusion. The planks being uneven wouldn't bother me too much as I would put it down to character. The deck has a nice aged colour that would brighten up with some fresh varnish. It is easy to over-restore things sometimes and you might find sanding those line out harder than you think.
Personally I would stick to two sails (Dragon like) for simplicity and a new mast will be easy.
The metal edge could be replaced with wood and the hull painted with a contrasting colour below the waterline.
It looks like it is missing a rudder. On the Dragon it is hinged on the keel but on your yacht it looks like it is further aft. That is a good position although you will have to make a new one.
It is a good question from your grandson and as a traditional pond yacht you adjsut the sails and the rudder and set it off across the pond. His young legs will run round and catch it and send it back.
Late you could think of vane steering or even radio controlled which would be very good. That can be a discussion for later.
A nice little project.
Tim R