Connecting an Apple II keyboard to a computer with USB is surprisingly easy with a Teensy board. The Apple II uses an ASCII keyboard, which means that rather than returning scan codes, it returns a 7-bit ASCII value. This also means you can’t read the state of modifier keys like shift or control independently. The Apple II keyboard in particular doesn’t even support lower-case letters (though I’ve made a bit of a hack for this). They even re-use a couple alpha keys for other characters, so shift-P makes @ and shift-N makes ^. Other late 70s/early 80s home computers like the TRS-80 had a really simple layout like the Apple II’s (though the TRS-80 had all four arrow keys but no Control key). I may have to try out adapting some other weird old home computers as USB keyboards — the C-64 seems like it would be a good shape/size for that.
Arduino (Teensyduino) code is available at https://github.com/afiler/keyduino.