nmerrill, the keelson is the bit of wood that runs the length of the canoe on the inside bottom. The keel is on the outside. I have no doubt the keel itself was a replacement. The two "floorboards" on either side of the keelson were atypical, and probably later additions. It is also possible the keelson was replaced at some point, but I can't say for sure. I've attached some photos showing how the Square End canoes typically look. Sorry they are on the small side.
Steel fasteners are not all that uncommon in old canoes (not desirable from a restorer's standpoint, but common nonetheless). Wartime canoes especially have lots of steel, as the brass was diverted to the manufacture of munitions.
Blue, since the keelson is such an important structural part of these boats, I can't imagine not reinstalling the keelson prior to canvassing - how else do you fasten the ends of the ribs properly? The keelson holds the keel in place, not the other way round...