Birch bark canoebuilding with no Birch bark....?

You could possibly use hemp twine instead of the spruce lashings? Or rawhide? Traditional kayaks were made from sealskins, so would some type of treated leather be an option for you? Birch bark canoes have been replaced by canvas canoes, even among native people, so that material seems to be the most likely substitute. It is quite a project - such a canoe will be a challenge to build. Best of luck with it.