Are the pins/bushings and rails worth a new set of shoes? Have you thought of welding on grouser bars? Are you buying a used set of tracks with 24" shoes?
22/24" shoes should do just fine for pulling a scraper.
In general it is advisable to keep the track shoes inside the scraper cut so that they are not riding on the berms.....so it would seem your 24 in. option would be preferred.