I'm just wondering how the threads got damaged on the pump shaft. It would be apparent someone was in that pump before. When you get the Key make sure you look and check the Hex Bolt and were the key goes. That is what going to take the most damage. It's not made any were near the grade or metal the pump shaft is. It would be hard to believe the Hex bolt keyway isn't damaged.
Check the Hole in the end of the shaft when putting it back together. It's there for a reason. Most likely takes allen wrench. Or did at one time. Just because if you shear the key you need something to hold the shaft and get the nut off. Or risk the damage just like what happened to the threads.
Oh no at least your learning. I just worked in a machine shop and multi million dollar printing equipment for 35yrs.. There's a lot who just will never learn. LOL.
I hear ya ! By the time I get this all fixed I will be the hydro master. I can't tell all you enough how much I appreciate all of you helping and taking the time to help a newb small farmer tractor owner!