Attex came from the factory with 21" tires at one time, and 22" sometime later. Either should be fine as long as all six are the same.
I've found some transmissions with the ground-off 18-to-9 tooth shift spools when I've torn them down and even had to reuse one in an emergency situation (didn't have another one). The machine ran for a long time like that and it did fine. I sold it quite some time ago and it's probably still going strong. Just letting you know in case you can't find a 9 tooth.
Good luck on the project.
Stuck in the seventies- not in the swamp.
(6) Attex, a Hustler, a Super Swamp Fox, (2) Tricarts, (3) Tri-sports, a Sno-co trike, 3 Dunecycles, and a Starcraft! ...so far