As long as the thrust bearing is made out of a good metal IMO(cuz they go under a LOT of stress), and the outer diameter will fit on the shaft hole in the motor mount, cuz some mounts have fairly big shaft holes like in the pic.
What I did to that problem is I put a small metal plate on the front of the mount and carefully drilled holes for the mounting screws and the shaft to go through. The shaft hole was small enough for a shaft to go through and a bearing to fit on.

Thrust bearings are not absolutely needed, but they can really save the life of the motor bearings. Also if it's set up right, it can reduce the damage from a an accident where the motor endbell pops off.