I think it'd be easier to variate the displacement by changing the bore other than the stroke. People forget that the stroke is in the crank, and since the crank is always moving, it'd be super hard to have any kinda changes happening. The bore is in the block, and doesn't move anywhere. You could have some sort of setup where the cylinder sleeves get bigger or something, but the pistons would have to change size too, so maybe they could be bigger but only have the combustion chamber as a smaller size so it would only have a smaller surface area acting on it. I can sort of see it in my head, can't see how it'd work though....