The cable is an 8 Pin DIN. AFAIK the cable is still available:
Cable - 8 Pin Din, Male/Male - 25 ft.
The FS has 7 LEDs so my guess is all 8 wires are used.
You can use a bunch of individual foot-switches as well. These eight 1/4" jacks are referred to as function jacks in the manual.
From the manual:
The eight jacks on the TITAN respond to tip-to-ground “latching type” logic which is a common language to most master switching devices on the market.
I would be surprised if the Footswitch works any different than the function switches. So, AFAIK, the resistors will be determined by the characteristics of the LEDs you use and the voltage supplied at the terminals of the foot-switch jack.
This may give you a hint for the OD, Solo, and Mute functions.
Obviously the A/B In and Channel 1/2 selectors would need a different type of circuit, but I would expect the limiting resistor to be the same value for all circuits.
Since the product is discontinued, I would call Mesa and ask if they can provide guidance or schematics of the foot-switch. I believe you will need to leave a message for a call back.
Good luck!