It wasn't me that hacked the cassette player, it was Smojo as I say in the credits. But, I think it kind of goes like this: dissconnect the motor, you don't need it. Fit a 1/4" guitar socket somewhere. Dissconnect the wires that come from the tape heads and reconnect them to the 1/4" socket. That's about it. If that's not right, my apologies. Even though I've not yet had a go myself, I do know it's pretty simple in theory. The tricky bit, I believe, is just getting at components, and u won't know what you're dealing with until you get the back off your player. I haven't checked, but there may be a hacking group on the Nation somewhere. All the best.
Thanks for the comment on my first kalimba build. Yew the metal is for pinging. The metal strips are played with the thumbs to get a music box type sound from it. Here's a link to a video showing a talented player playing a kalimba. (It's not me playing.)