it is not really a "modification" to add an FM RF amplifier stage, heterodyne/mixer, and demodulator then couple the audio to the existing output stage. This is a wholesale redesign or actually, adding a separate FM radio to the existing case. Adding an auxiliary audio input is much easier. they make re-pop radios that look original and are all solid state (of course) and about the size of a stack of business cards. they use way less power too. only problem is they are $500...
