Each FET has a pulsed current rating at a certain temperature and also a continuous drain current rating.

Say each FET has a continuous drain current rating of 40A, 40x4 = 160A so the manufactures would rate the ESC at around 160A or less to allow for temperature loss. 160A is more than enough for a modified motor, now half the FETS from 8 per phase to 4 per phase (2 FETS per polarity phase ie 2 FETS for the positive A phase and 2 FETS for the negative phase) then your continuous drain current rating of 160A will change to 80A. This in turn determines the motor you wish to use with the ESC.

In your situation I would give the ESC an easy life with a high turn motor like 21.5t and cut your losses. You are very lucky it even still works.