I don't know why that pin would be hardened. It's only function is to keep the idler shaft from moving out of position, and a dead soft pin should be able to accomplish that. The fact that it's mushroomed seems to show that it's not hardened. But you'll find out once you try to drill it. Centerpunching it would tell the tale as well as to how hard it is. I'll bet it wouldn't take much of a tap on the punch to put a nice mark in it for drilling.
