Hi PawlieWith regards 5th gear you will need to change not only the 5th gear but also the 5th pinion gear some time's called a dog gear, these are mated parts and when one wears the other wears, you will probably find wear on one side of the teeth on both gears. As you have the engine a part it is worth checking the other gears and the amount of play and wear in the gear selectors as well as the shift cam, as these do wear and break, i had the 5th gear selector go on a trip in Morocco it was a long slow ride back to England.
As the others have said up grading the oil pump is a good idea, as well as just giving all the internals a good check over, bearings, shaft's, oil ways and the casing's them selves. Have fun Skip
Do the best you can with what you have,
A stranger in a strange land now heading North South East West to.....