A tooth can be formed by adding material by edge welding with a mig welder and filed back into shape. I have done many projects like this, just find a competent welder or practice on some scrap metal of similar thickness first.

If your willing to test out the possibility, move the mechanism by hand into the next position and see what happens.

If its that tough then i question that it was worn off. I had a machine that someone removed the credit mech. and in order to stop a counter from applying a credit, they purposely filed off a tooth. Since he can't get to work on the machine properly at the moment, he might not see the big picture. Maybe there is more to the story of the missing tooth.