Most GE Elec-Trak drive motors are Separately excited, some are PM type.
This is the Alltrax controller that is recommended for the Elec-Trak with SepEx motors.
Electronic Controller - $418.00 : The Electric Tractor Store :: Elec-Trak Parts
With the manual and Wiring diagram.
I am currently doing a Alltrax conversion on an E15 tractor.