I doubt it can be fixed. What's missing is completely flat and somewhat thin. I've never seen a brass of any composition that would lend itself to any type of repair. Except it can be drilled and tapped. So perhaps I can make the missing piece out of screw stock and thread it into the main body of the firing pin. That just hit me as I wrote this. I'll be in the shop once I get home as I'm out of state again. Thanks for kick starting my brain. It doesn't happen often enough.
$175 for a firing pin? Mister English thinks quite a bit about his work doesn't he?