Agree with Joe. Hopefully there are a lot of good threads and just the first few are damaged. I can't tell by your description how much is damaged.
Anyway you can use any number of tools to rebuild big threads or cut them off if there are enough good ones to use with a thinner nut. You are not in big trouble. It's plenty fixable. Determine size of threads. Need to find a nut.
To rebuild threads, assuming you don't have a die that big, use a hack saw blade, files, chisels.... (maybe a Dremel) to reshape the threads. Big threads can be done with those kinds of tools. You don't need the first few threads to be good, they just need to accept the nut. They are not part of the torque strength. The most important thing to remember is that it will take time, patients and good lighting. Slowly but surely you will get it done. You really have nothing to lose if you can't make it work. A new rethreading die is only $10. Guessing 3/4-10. (Home Depot). You don't need a good one. You won't use the die until all work is done with tools to reshape threads gently....or cut a few off and rechamfer the end. As I said, I don't have a good feel, or pic, for exactly the extent of damage.
A donor gear box can be $0-$100 and that shaft is all you need from it.
Of course, a machinist can fix anything.
