Hydro Flame furnace main burner not lighting

Don't do a gas valve replacement before you check to see if the furnace burner itself is so plugged with wasps nests that it simply can't let the propane out to burn. Wasps love to build nests in these furnaces. I once removed a furnace from a camper and got a coffee can full of wasps nest out of it by shaking and digging around inside the plenum .

P.S. you might find out that you can not get parts for a furnace this old.