If you have it out as shown in the pictures, there's not much you can hurt by taking the end off the trans to make sure nothing has been cooked by low oil. In my experience a large amount of 020 transmissions are changed due to this. At this time you can test fifth to make sure it's working and even adjust it if necessary. If the teeth on usually the driving gear are gone, I'd suggest another tranny is needed or have to pull it apart and get all the teeth before using it with a new 5th gear!