How do I replace the

Your best bet is to do the whole thing. If the bendix is bad then there should be undue wear to the shaft. Not to mention that the starter is old and had a lot of use. I have been a mechanic for 25 years on European and exotic cars and I dont rebuild starters any more because they dont make them like they used to. They are ment to be a trrow away item noe and have for 15 years in most cases.