Thanks RCP' - last night I fitted the 400 brushed.
Yes, I had to file the motor-plate and also the little holes with sand paper, took a while but now it fits nice, ratio is 62/15 - I had to hammer a plastic pinion on. Lol.
I might make a video of it and update this thread because it certainly moves a lot quicker than stock despite the weight of the motor and the nimh battery. (7.2 volts).