A lot of Johnson's defensive tactics relied on him being stronger than his opponent and thus able to tie up. I think he's smart enough to adjust to a bigger, stronger fighter (Norton was often called Black Hercules, and for good reason). I'm aware Jess Willard was both of these things, but Willard was also a chump who caught Johnson on the tail-end of his career.
I think we'd see a different Johnson to any we'd ever seen, but I'd still pick Norton by convincing UD over 15 rounds. Johnson has the advantage in any fight longer than that though.