BTW, should I even ask about replacing the HX or is that completely
ridiculous on a 26 year old unit? Actually, over the years I replaced a
gas valve, inducer motor, and inducer control board (new one has the time
delay) myself. So aside from the blower and a small circuit board, it's
just a box to hold these components, right?

I'm hoping someone is offering one with spark ignition. The more I learn
about hot surface ignitors the less I want one since I know it's going to
fail.