Well, it's not too difficult, but does require a bit of work.
You have to pry the white connector shells off the 3V board (except the battery connector) and replace them with the defunct 5V board shells. Heat helps get the shells off without damaging the board. Note that you have to take great care NOT to damage the PCB traces on the 3V board getting those shells off. Press the black 5V shells onto the 3V board, the board is prepped.
You'll need a different heatsink, or fabricate one from aluminum.
You have to replace the 9V battery & harness with a 3V battery & harness, or just use a 3V supercap replacement with the connector for the PS/2 battery connector.
Replace the 16 ohm speaker with a 4 ohm speaker.
Mount the 3V board, plug all the existing harness plugs in (except the 9v battery).
Connect your 3V battery or supercap and you're ready to go.
