I've seen that sort of wear when the front output shaft bearing has gone, and the gears don't mesh properly, along with grinding all the swarf floating around in there. It might not hurt to re-use the gear, I don't know. But if another better one was available, I'd certainly change it out!
It's actually best to use fifth as an over-drive and not put much stress on it because of that front output bearing.
Wanted: Dead, smashed, crashed or trashed gauges
BMW GAUGE REPAIRS - TACH*SPEEDO*CLOCK*VOLT METER *PODs & LIGHT BOARD*