Does anyone know who can test and or repair stock AM radio?
Honestly you should consider leaving it looking stock but having the internals replaced with a modern Bluetooth compatible HD radio setup. See the West Coast Classic Cougar website about the service. It will look exactly the same but you will enjoy it much more.
Scott’s Electric Service in SC can restore it to its original condition. Not many people doing them these days. They did my AM for $300 or so. PM me if you’d like their contact information. Make sure you have a good core, clean case, nice lens.
Also, a company called Electro-tek makes the correct ohm speaker for the dash. It will sound better and last longer with the right speaker.