To replace, use a stainless socket head screw, there's nothing special about the bolt. There is a crush washer beneath it that is important.
To remove that, you've got a few options:
- easiest to me is to weld a short bolt to it and then use a socket or hex wrench to remove it.
- heat the ever-loving dog piss out of it first, and I mean cherry red, then insert a square drive extension into it and turn that out with a ratchet.
- drill it out with left hand drill bits.
That bolt connects to the damper rod internally and it will spin endlessly if, the oil is emptied and the spring/top but are off the fork already. Use a wooden broom handle wedged inside to hold the rod while you remove the bolt.
Also, use a dab of Antiseize when reinstalling the bolt on the threads.