while all 'technical' suggestions & insights make senses and sound appropriate, let me point out rather a sentimental & spiritual & religious. (aka perhaps useless).
drilling out a track fork—especially such vintage ones with tight clearance—always feels me a bit of blasphemy. i dunno.
i'd definitely try to fill the hole and naturally repaint—paint & rust looks bad already + brazing/welding will most likely hit the paint—like nothing happened, saving the soul of such gorgeous vintage pista fork. then a Keirin brake clamps—seller called Track Supermarket also carry a few kinds other than the ugly Dia Compe Spcaceship Alien Bionic Limb Simulators, IIRC.