Sounds like a bunch of rental canoes I saw years ago in Wisconsin. I suspect they may have even used an old canoe for their mold or plug. They looked like someone had laid up a few layers of glass with an untinted resin, and then added gunwales.

Unless you already have a surplus on hand, I don't see why you want to spend the money for epoxy resin to fix this. If it is polyester you can patch it with polyester. If there are a a lot of fixes to make it would be much cheaper. Polyester is much more tolerant of UV than epoxy, so any repairs you do would not have to be varnished to protect them.

Birchbark canoes were made by pulling the pliable bark into the rough shape of a boat, securing it between gunwale strips, and then forcing in ribs. Sounds like you are saying that this is what they did here. The basic structural concept is sound and time-proven.

Patch away on the "skin" of plastic-reinforced glass fiber. Anything that keeps out water is good enough. It is not giving you any strength. The ribs and gunwales provide all the stiffening and strength. Replace these components with new wood and you'll have a nice, modern take-off on a birchbark canoe.

Any new wood you install: once you have the right size, coat each piece with two coats of epoxy resin and 2-3 of varnish before you install it. That should make the wood rot-proof for decades. Make sure the gunwales are right first, then you can add ribs, either on the spacing they had, or you can spce them closer together.

That thwart was not just there for portaging. It (and/or some seats) gives the boat enough resistance to water pressure to keep it from folding in on itself when heavily loaded. and it keeps the ribs from straightening out too much when they are inserted.

PGJ