Hello Paul,
I'm pretty sure the standard Myford change gears will suit, I don't know about the bore diameter or linking detail but suspect the Zyto equipment was mostly a crib from the Myford ML 2-4 series. For reference the Myford gears are 20 DP with a pressure angle of 14.5 degrees [as they mostly were in those days] with bore diameter of 0.625 inches. Linking was by a small diameter pin in the hub, today there is a 1/8 inch wide keyway cut in the bore, but plenty of room for a pin nearby.
From about April next year I will have either a Crompton or a good Gryphon 1/2 HP single phase motor 1425 rpm motor surplus for purchase, send me a PM if you are interested.
Regards
Brian
Edited By Brian Wood on 23/12/2018 10:08:53