Phil,
Parts number 20 and 21 are for a closed canopy (as I expect you already know). In a pinch they could substitute for the open canopy pieces #23 & #22. You could still use them, in your case one, to replace the buggered up piece. A little sanding around the outer edges might be needed to enable the "replacement" part to allow the sliding canopy to clear it. Just a thought.
John