I would start over with an entirely new power-supply, if you are so-inclined. You will get better reliability, at least, than dealing with 20-30 year old components.
https://www.teslaboys.com/Plasma/PlasmaPower/index.html
http://amasci.com/tesla/sstesla.gif
....of dozens of possibilities.
If your flyback is good, you may only need to "fix" the driver section.
Peter Wieck
Melrose Park, PA