Why not consider finding a '69-70 AM radio and converting it to FM, with MP3/Aux. inputs, and replacing (and storing) your original ? I'd also consider buying one of the reproduced AM/FM radios, as they have a modernized chassis that accept most of what you would want to enjoy a good system without sacrificing the original appearance.
Then again, you can always carry a separate sound system that you can remove when you want to. All of mine are AM's, I'm faced with the same dilemma - I want the appearance, but I need the sounds.
Regards,
Steve