Progress is being made... The boost converter now works again!
I replaced the inductor but didn't really expect it to fix the problem, since it didn't test short or open - and no, it didn't fix the problem.
So then I decided to just start replacing the only parts I hadn't looked at: D3 and D5.
Replacing D5 (60v Zener) didn't cure the problem - but after replacing D3 (MBR160) the boost converter is now producing 60v once again - see attached photo - no longer just a square wave!
Unfortunately, there is still no display after reattaching the MAX6921 and tube, although interestingly enough, the boost output drops only to 59v with the MAX6921 installed and 48v with both it and the tube... so perhaps it's a bad connection, the MAX6921 isn't doing anything\drawing current, and the voltage does not drop.
Will check to see if there's any signal coming out of the MAX6921, and I guess check all its solder joints if not.