Although I now know of other options (thank you, Tony Wright!) I had already decided on this approach. I am happy to say it works.

I used the old cast white metal frame as a go by, using the Romford screwdriver to register the rear axle and a gently placed round needle file in the front axle (I would have used a piece of 1/4" brass rod but had none to hand). I also kept the center axle on the new frame as a check. I then scribed the outline of the XO-4 mounting from the old frame to each side of the new nickel silver frames. The difficult part was measuring the spaces for the motor's front lugs.

The next job was to cut out the mounting - see the next post.