they said the wood is so thin that if I only cover the outside it might break when using the canoe. The only way how it might work is that I cover the inside and outside with Epoxy.
It's understandable that someone without much knowledge of wood-canvas canoes would think they are too delicate. But the builders didn't think so! And the fact that many of these canoes are well used and very old says that they work pretty well with canvas. A wood-canvas hull is surprisingly resilient. Check the comments in many other posts on these forums and you'll see that many users know these boats aren't dainty little delicate things.