I have the 10" version of this set, it's flyback was toast also, I'm assuming here that your set uses the same chassis with the Fly mounted on top?
Since it has the same Sweep tubes as the RCA 630 and similar I was able to swap in a common RCA FLY-1 flyback with no wiring changes, other than replacing the resistor network in the yoke to the RCA values, it works great, runs cool and is a much better flyback than the Zenith.
If I can find the pictures of the swap I'll post them.\
Ahh, found it.
And the thread about it:
http://www.antiqueradios.com/forums/...yback&start=20