if you look on top of the adapter that your bad gear slides into you will see numbers, those numbers reflect the number of teeth on the new gear your installing. set that number on the mark in the tail housing and tighten down the hold down clamp that's how you set the gear lash for your new gear.
As bad as that gear is I wonder if the gear in runs on is any good . My best guess is that the hold down came lose and allowed the adapter to turn ,as the gear moved away from the drive it stripped the teeth. It's all in the TM for better explanation