I might be wrong, but I think that division of Siemens was sold off and is now called Infineon. Not sure how much that will help, a lot of the old Siemens branded devices are now so old they may not be available, but they might be able to cross it for you.
" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden
If it looks like a one amp device, I wouldn't hesitate to use a 1N4007. If it looks smaller than 1 A, I would still use a 1N4007, if the leads aren't too thick. If so, I would go for a 1N4148. They are all much better than any of those old devices (are they soldered to one-sided with no through-hole plating phenolic paper boards?)
We once tried to get an exact replacement for one of those and measured another device on the same board. It was very difficult to find a modern device that was worse. Leakage current was terrible, Vth about the same. Those "secret" specs are nothing but incompetence trying to get well paid for.
If it isn't a radar application or other HF, just pick any diode with similar (or smaller) dimensions. Those metal can diodes with axial leads were not critical in any way.
Found your diagram in another thread. Are these the v1 or v2 rectifiers? Then you need three-phase bridges with rated DC output corresponding to the "wire size" (if you have nothing else to go by. Like a name-plate with excitation data un it.
You have tightened all Connections, of course? And checked the set-point potentiometer, if there is one, and the R2 (the ganged potentiometers)?