When I had furnaces I would all stock parts, fans, gas valves, ign trans, trans, brain boxes, thermo couples. I even have a gas valve to an old coleman mobile home gas furnace, guns to oil furnaces.
You will find out that furnaces break down late at night, holidays, when stores are closed, and the coldest night of the year.
Furnaces don't break in the summer, go figure.
I switched everything over to electric baseboard heat and rarely have a problem. If there is a problem, it's only in one room. No big deal. House will still stay warm
Sad to say, HVAC repairmen would be out of a job fixing furnaces, heat pumps, if everyone converted to electric baseboard