ill do some math on the calculator...
72/21x2.5= a little over 8.5. so id start with a 21t pinion but this is based off a 7,400kv motor. so i'd give or take a few teeth. dont go over 180F in heat or if you dont have a heat gun, see if you can hold your two fingers on the motor for 5 seconds or so.
TEAM HACK-HACK OR BE HACKED