Good evening David,
It certainly is possible to replace the EY51s by silicon diodes.
Or by new EY51s
I had the oil examined and it didn't contain PCBs.
The transformer coil is vacuum impregnated with the oil, so do not keep it dry for too long. It would be best to keep the coil in oil.
The capacitors are suspect as well, but there could also be a fault in the coil itself.
The repeat frequency of the ringing choke is c. 1kHz, and the ringing frequency itself c. 21-23 kHz.
Hope this helps a bit.
Jac