Mike...you are correct...that shaft is case hardened......don't think you will be drilling into it very easily.
.....I did see a guy who carefully, butt-welded a small threaded stud on the end of the shaft and used
a washer and bolt to hold the counter sprocket on. I thought that was rather neat. Doesn’t need a lot of pressure, ..no more that what a snap ring can take.
I'm with you Mike, just replace the shaft, I think George can get them.
Happy Holidays
Flipper