Hello Hazel,
That's quite a project for someone past three score and ten to take on - obviously you have more energy than I can dredge up at a similar age. I hope it all goes well.
As I understand it, the mainsail is good enough to be used, so the sailplan is fixed as it is. It's as much area as could be fitted onto the hull anyway, so the performance will be what it will be. Given new sails, I'd be putting in 10% camber, to get the area driving a heavy boat as effectively as possible.
I would use 3:1 halyards on all three sails, with selftailing winches, to cut down on the miles of string there's going to be. Size of winches matched to your size and strength, possibly 16:1 for fore and mizzen, 30:1 for the main.
Sheeting is tight, and will probably snag when tacking, but this is a boat that isn't ever going to short tack up a narrow channel, so that may not be too serious. The deck is big enough to walk around twitching the sheets clear of the batten ends. I'd want to put port and starboard sheets on the mizzen, to sheet it close enough for windward work, to act as a riding sail, and to reduce snagging.
With such tall sails, the middle of the sail tends to shunt fore and aft in a seaway, distorting the set. I think I'd try a second luff hauling parrel, at mid height, to keep some control over this area. Otherwise, the sailplan shows the normal YHP and LHP and this looks OK.
Isn't the plaque recording the registered tonnage? 1 register ton is equivalent to 100 cubic feet of internal volume, so the registered tonnage of Colne is about 19 ¾ tons (nothing to do with displacement). She must have had Part 1 British Ship Registration at some point.