I replaced the bad Flyback in my 10" Zenith Porthole with a common Fly1 from an RCA.
Details here.
It's a completely different chassis than yours and it was a Fly designed to run a 10-12 inch tube so I don't know if it would work here or not but possibly another fly for a 16 inch tube would work in this application.
What tubes does your set use in the Horiz output, this was a set that had a 6BG6 output, 5V4 Damper and a 1B3 Rectifier, the only electronic mod I had to make was the capacitor inside the Yoke.
I have an HVO-5 for a 16" RCA, it looks about the same as the 10" Flyback but the windings are fatter, it's for a 16AP4 or similar tube.