The HV filter cap is a ceramic type and is not too likely to be bad...it is 500 mmfd (micro-microfarads, or picofarads) not 500 mfd.
Here is a source I have found of these type caps:
http://www.rfparts.com/caps_ceramicdoor.html
If you search for "doorknob capacitors" you will probably find others too.
Regular use does seem to help old electrolytics stay in better shape...I would still replace them myself but if you are going to keep them in it would be a good idea to use an ESR meter and/or capacitance meter after the caps are properly discharged to evaluate the status of the caps, as they do continue to degrade over time.