[Marshall V. Daut]

You might try matching the broken ring at a hardware store to see if they have a snap ring close to that size. As long as there is some kind of retainer on the bottom of the steering shaft in that groove for the worm gear to butt up against, the gear will not slide down. It's a tight fit anyway between the gear and the shaft. The pressure exerted by the worm gear being pressed against the ring will keep both in place. You might end up putting two thin modern snap rings in place to match the thicker original one. Depending upon the type of modern snap ring you find, the "ears" with the holes for the snap ring plier jaws may need to be ground off so that the outside of the ring is round all the way around and will fit inside the worm gear's groove.
I am surprised that given the number of parts available from vendors to rebuild the Model A seven-tooth steering box, this little guy isn't also offered. It's almost a "must" when rebuilding this steering design because it's difficult to remove the original ring without breaking it.
Marshall