Colin,
According to lathes.co.uk the ML7 change wheel assembly can be grafted onto any ML4 which doesn't have tumbler reverse. This might be your simplest route, if you have deep pockets.
If you want to use the ML4 set-up here are some dimensions taken from original parts. They are in the native English units which would have been used when they were designed in the 1930s.
Drive pin: diameter 3/32", length 15/32", material silver steel.
Drive pin hole in change wheel: diameter 0.096" (No.41 drill), depth 1/4" minimum, location 13/32" from change wheel centre. Hole diameter must give clearance for the pin, and also allow for any misalignment of the matching hole on other wheels. The depth can usefully be greater to allow for the build-up of crud at the bottom of the hole.
Drive collars: These transfer the drive from mandrel to gear train and from gear train to leadscrew. They are 3/8" thick like the change wheels, and 1" diameter. The drive pin hole is located as for the change wheels (though on mine the pins are a press fit and are not removable). There is a 1/4" BSF x 1/4" grub screw to secure the collar to the shaft.
Retaining the intermediate change wheels: I have owned two ML4s. The earlier one has plain studs for the change wheels, and needs plain collars with grub screws to retain them. The later one has cross-drilled studs, and the change wheels are retained with split pins.
George