You can't readily just remove the canvas and put it back on. The fasteners that hold the canvas to the hull are right at the very edge of the canvas, so there would be no way to effectively reattach the canvas and have it be tight. In case you haven't canvassed or read about canvassing a canoe before, the canvas is substantially larger (longer and broader) than it will be in its final form. It gets stretched on very tightly, tacked in place at the sheer line and at the stems, and the excess is trimmed very close to the fasteners.
As for replacing ribs and planking while the canvas is on, that's impossible if you're to do it as original. Even if you can get the wood into place, you won't be able to hammer tacks through planking and ribs. Of course there's always a way to effect repair of damaged parts with canvas on (Tom described one method above), but you just can't repair it in the original building manner without removing canvas.