I'm holding off on working on the cabinet for a bit because of the weather.
I did get a replacement flyback though. It is a Merit HVO-53. Came with instructions. Nice.
I did have to drill/file new notches to mount the new flyback.
I followed the instructions as to the wiring and reassembled the HV cage.
Then the moment of truth. It works very well. I'm getting 12.5Kv and it is dead silent! No more crackling sounds.