Need advice regarding a regulated HV DC power supply for Magnetron
The simplest solution to this problem is to replace the existing "Villard Circuit" pulse-type voltage doubler with a voltage doubler circuit that outputs solid DC.  This modification is significantly less expensive than a new power supply.  It requires only two appropriately rated diodes and two capacitors.

Two safety related modifications are also suggested.  First, a small bleeder resistor should be connected across each capacitor to drain away charge when the device is not in use.  This will reduce (but not eliminate) the possibility of an accidental discharge.

Second, a fuse or circuit breaker should be installed if not already in place.

Finally, to round out the updated power supply, consider adding a cooling fan and voltage/current meters.

Special thanks to @kalleMp for inspiring this answer.  I am in their debt.