It looks like the motors are 1.2Nm holding torque so with a ratio of 7.2 that would be about 8.6Nm of holding torque, so the actual torque running would be about 4-6Nm depending on the speed /voltage.
You are right, you will need to dump the existing X and Y motors because of the gearbox, not specifically because of the max speed but also because of the 45arc minutes of backlash and that the system uses belt and pulley so that 45 arc minutes would equate to quite a bit of movement.
I would personally use a mid sized 34 frame motor if you are going to continue using the existing belts or going to a 60mm diameter (3Nm) motor if converting to ball screw.
I would also consider reducing the height of the router to make in a bit more ridged.