I have a Minn Kota Fortrex foot control. It is located in a recessed tray. The rocker switch for momentary/constant is on the side and cumbersome to get to. Can a stomp switch like a big foot switch that is made for Hand Control trolling motors be wired into the Fortrex?

Are you wanting to turn the motor off, turn it to constant, turn it to momentary -- I guess I'm asking what you're wanting to do. To answer the first part of the question, a big foot style switch can be put in the ground side of the wiring to turn the trolling motor on/off, but if you're trying to go to constant then momentary, I don't know how one "on/off" switch would accomplish this.
John

Basically, I'm wanting the constant/momentary switch located on the deck of the boat, in lieu of on the side of the foot pedal.

Seems like (I would have to check to be sure) the old Motorguide "stomp box" had a two position dimmer type switch that was set up with a 3 wire connection -- which is what you would need. It would be an On/On switch with no off. The foot switch like the big foot would not work in this case.
John