What would be a suitable replacement for a selenium B+ Boost diode at least for troubleshooting purposes?

Others may have a better idea, but I think an NTE580 (fast-recovery, 3 amp, 600V) would be more than sufficient. An NTE506 (0.5A, 1500V if I understand it right) may be suitable as well. The important thing is to use a fast-recovery diode rather than a standard 1N4004-4007 or similar.