You can make changes to a design, but the wise builder does so after consulting with the designer or by making similar substitutions (similar physical attributes). In other words, you can substitute pine for spruce, but it wouldn't be wise to substitute pine for oak.
In regard to changing scantlings (the dimensions and/or specifications) of things, again do so with care, if not designer approval. It's really easy to screw up a design making what seem to be innocuous choices, just to find you've created dramatically increased stress risers or localized loading issues. Most novices and backyard builders haven't a clue about engineering cantilever beams or point loaded structural elements, so it's best to stick with the plans.
Most canoes and kayaks are covered with light weight cloth. 4 to 8 ounce (140 - 270 g/m) 'glass cloth for sheathing with either regular cloth (not very strong) or biax (preferred) tape for the seams. Again, it depends on the design.