The first thing I would do is isolate it to a power supply issue vs a radio internal issue. I would take a voltmeter and check for 12v at the power plug into the radio to ground.
Pull it, find a company on the net that does am/fm conversions to old stock am radios and send it out...of course if the budget allows. Had my original am done years ago and it still works although i have forgone using it for an under dash cd player.