Mark,
I agree to start with a tiny triangular file. I see thread damage on the starting threads to the right of center in the (very nice) picture. There's damage further down as well. Also, grit in the threads and what looks like hardened sludge in the sealing washer area.
1"-20 UNEF (extra fine) nuts and dies are available, but probably not locally. You can take a
1"-
20 UNEF nut and make several hacksaw cuts in the threads to make a thread chaser. That nut will also let you know where the threads are damaged as you screw it on.
1"-
20 UNEF dies are also available. The type of die with a screw adjustment (for thread depth) can serve as a thread chaser in its loosest position.
1"-
20 UNEF bolts to check the threads in the sump will be much harder to find. If you buy the
1"-
20 UNEF die, you can put some threads on a piece of 1" round stock to check the threads in the sump.
Good luck. Some sources below.
https://www.scagparts.net/hydro-gear-51821-1-20-slotted-hex-nut/
https://www.amazon.com/Universal-Outside-Thread-Chaser-Similar/dp/B07XGQQSY6
https://www.mscdirect.com/product/d...2ODkLjlIUOviwtPM-9xbozqJulHEvxMyXlj98oGcHFuP4