A lot of older radios with an FM or VHF band on them have been given a new lease of life with the advent of plug-in FM transmitters for mp3 players. For older, but still collectable, radios which predate FM, my understanding is a lot of folks add an FM band by conversion, but there surely must be a market for a similar, small-reach transmitter device that AM hardware can pick up... cheap mp3 player (or tablet) with downloaded content of the right period, and away you go.... I wonder if anyone has made such a product yet, or is it the case that FM is just too dominant?
The definitive small AM transmitter in the US for several years was the AMT-3000 from a company called SS Trans. It was sold as a kit, and you needed to be handy with a soldering iron to put it together, but once assembled, it outclassed not just cheap novelty transmitters but many commercial broadcast models. The guy who sold the kits went out of business several years ago because he couldn't keep up with the demand. There is a company in the UK making a similar unit under the "Spitfire" brand, which is well worth considering. These are easily found on eBay and while not cheap, will be a satisfying unit to anyone who has a good AM radio they want to put into actual use. There's no need for Bluetooth or any of that -- these transmitters all take audio thru RCA plugs connected to a headphone jack and operate over about a quarter-mile radius.