This is a pretty unique footswitch. Your best bet is to bite the bullet and buy one from Mesa.
Check out the schematic, which you can find for the amp on the web -- this includes footswitch info. The footswitch uses a multipin connector. As I recall, the channel switching uses two of the wires with different resistors to switch channels. The amp senses the voltage (depending on the resistor selected at the footswitch) and decides which channel the voltage represents, and does the switching. The Solo and Reverb switches use some of the additional wires off the connector. Finding a multipin connector and wiring all the LEDs, switches and resistors together in a nice box would probably not be worth your time, unless you are good at this sort of thing and have done something like it before.
