Not sure about the ZX centre ball diff, but then I ran mine with the balls removed so it became a crude slipper as the diff sending all the power to the spinning front wheels wasn't the best design idea in the world. Fitting the reverse one way rear drive hub on the layshaft would also sort it, but obviously the cheap option was to leave the balls out.
I know the ZX-R should handle brushless, my runner has been totally reliable (ignoring accident damage!) with 10 - 12 turn brushed motors in it for some time now.