There's a big community of ambient/electronic musicians that are still using these things. They are popular for recording/playing tape loops and making tape delays. I bought some recently to convert into a makeshift mellotron (they still run ~$15 on ebay!). [1] Also there seems to still be enough demand to fund kickstarters for new fancy cassette players. [2]
I still have a fondness for how the walkman's motors would slow down as the batteries died which caused a slowing of the sound. The beauty of analog. Today's digital equipment stops cold once voltage gets below threshold.
Along the lines of the melotron, I wonder if you could hook up a pot connected to the battery inputs to manually adjust the voltage so you could control this slowing down effect. I could see it working to lower the voltage, but would these things speed up by applying a higher voltage than normal?
[1]: https://schollz.com/blog/tape-synth/
[2]: https://www.kickstarter.com/projects/wearerewind/we-are-rewi...