GetLoFi posted detailed info on using their LTC1799 Precision Oscillator kit for pitch control on a Casio SA-39 Keyboard –
The kit oscillates at frequencies from 1KHz to 30Mhz and its output frequency is controlled with a potentiometer. The oscillation frequency from this kit can be substituted for the constant oscillation frequency a device receives from its internal crystal. The kitâ€™s variable frequency will allow you to control the pitch of a device when it is patched in place of â€“or sometimes in parallel withâ€”the deviceâ€™s internal crystal.
Likely interesting even if you aren’t working with that specific hardware – Circuit Bending Casio SA-39