If you actually broke off 2 teeth I wouldn't run it you risk throwing the chain off. If it's just a 12 tooth to begin with you're fine. If yours was a 14 you'll gain a touch of accelleration going to the 13 and a little top speed, you might also add some tire spin and in return loose some acceleration though. If you had a 12 you'll loose a touch of accelleration going to the 13 but gain some speed, and it should slip less.