Again, I'm def no expert, but I think hardened steel can be milled/machined.
The proper way is to machine it in the annealed or normalized state, then heat-treat and grind.
Hard milling and hard turning are fairly troublesome.
I've done some parts for my other bikes...
Of course, now that I'm into this KX500 thing, I'm considering manufacturing certain gears and forks here in my shop.
Here's an older 1st attempt prototype shift fork for a YZ tranny upgrade I run in my own bikes: