You could just take the end cap off the gearbox and have a look. If its just a case of a broken cog, but all else seems fine, then change the 5th gear. Can be done with gearbox still in the car and you can drag another set of cogs from another gearbox in the scrappies with a taller 5th gear. This way you will decrease the revs whilst in top gear, giving you better return on your MPG as well.
Obviously though, if the cogs are mashed, then there will be a load of debris floating around the gearbox, in which case, a swap is on the cards.