I have a 1950's furnace (believed to be circa 1954 , unknown manufacturer)with a "Penn Type 520 - Model 1000 Combination Furnace Fan & Limit Control".The fan motor runs continually . After checking all of the connections(for rodent destruction),I believe the fan control switch has gone bad. My question is: Can I retro-fit a newer switch to the chassis of the control or will I have to replace the complete assembly?