After looking on ebay for possibilities it became clear that it would be too complicated (and destructive?).
I did some research and found these 2 items on ebay:
a bluetooth adapter in a tape cassette.
http://www.ebay.com/itm/322385616259?_t ... EBIDX%3AIT
and an mp3 player (with remote) in a cassette.
http://www.ebay.com/itm/252509649751?_t ... EBIDX%3AIT
The next problem was when I tried to play a tape (it would not move). If it would not move the mp3 and bluetooth player would not work. I found that the source capstan was almost frozen and the take up was stiff. Some judicious applications of light oil and some moving things back and forth freed it up enough that it played a tape just fine. Pretty good for a 32 year old boom box.
The next issue was that when you pressed play on the tape there was a loud crashing static sound and one of the channels (usually the left one) would not work or fade out after a while. I was worried that there was a problem with the amplifier or maybe the play head on the tape recorder. I opened the unit up and was surprised at how easy it can be worked on. 7 screws hold on the back, the power supply is in the back panel and is plugged in to the mainboard, and there is a connector for the speakers.
I reassembled it, and tried the mp3 and bluetooth players and now there is no crash when I hit play and neither channel fades.
She sound great.