So after a lot of searching I’ve managed to get two old tape players to use for parts. One, which can only read, I bought off of ebay for a few quid (and is yet to arrive) while the other, which can read and write, I got from a lovely couple on free-cycle.
I managed to find the service manual for the tape recorder which was very useful as it allowed me to tear it down without risk of damaging it. As it turns out it is almost perfect for my uses.
The design of the recorder is such that I can remove the cassette assembly as one piece and simply transplant it into my own box. In fact it is so simple that the only IO I will need is the motor controls, the tape head signal and the switch indicating that the play button is pressed. What makes it even simpler is that all of the controls are dealt with mechanically. The fast-forward/rewind buttons change the gearing of the mechanics to increase the speed of the tape – no electronics required (although I will want to control it electronically, it is still one less thing to reverse engineer). The pause button simply disengages the tape-head – no electronics required (this will work nicely as a mute button). I could go on about how perfect this is, but hopefully this will be self evident when I write about how I actually use it.