Mouser and Digikey stock R5000F diodes. Their modern diodes with 5KV PIV rating and 200ma forward current rating one or 2 in series is my usual replacement for focus rectifiers....I've even put 4-6 in series and used that as a solid state HV rectifier before.
https://www.mouser.com/ProductDetail...NU2tEYAg%3D%3D
If focus is low I usually put one in parallel with old diode and see it focus voltage improves. The old selenium focus rectifiers tend to fail by increasing their internal forward resistance.