So does the starter grind/make horrible noises or everything functions correctly and you're just worried about it only engaging halfway? Cuz if it were a 168/153/wrong starter thing, it would make all kinds of terrible noise when trying to start.
If it's purely just engaging half the teeth but working fine otherwise, I'd contact that guy at American Powertrain back to see how his is but honestly, I wouldn't worry about it. I've had starters in the past that didn't quite get 100% engagement and worked fine for a long time.