[heatmon]

Can you be more specific on the choice of fan & limit switch which you replaced yourself? Did you replace with a cam-stat (original equipment) or the honeywell recommended about 20 years ago, as this furnace could be almost 40 years old. Late 60's to early 70's as I remember. You didn't state the exact problem you're having, and your furnace never did have a fan relay, unless it's handling air conditioning in the summer. If so, what happens when you turn on the fan switch at the thermostat?

This is just my opinion, and with it and a dollar, you can get a cup of coffee in most places.