OK, bit further along with this.
First off this is what it looks like in the high ratio for screw cutting.

Simple three gear drive and all the gears on this lathe are 8 shallow splines and special to Colchester.
The spline is 4.36mm wide measured on one of the gears and that's not a standard size in imperial or metric but a while ago I bought a job lot of broaches and bushes and in this was a broken 8mm broach missing about the top 1/3rd. So bang it on the surface grinder and take both sides down so I finish up with a parallel broach at 4.36mm wide. Broaches do not have side relief, only front clearance that that works out well.
Next job is to make a new bush with the OD to fit a gear's ID and a 4.37mm slot in it for the broach. Whilst this is set up in the dividing head on the mill, the bank is rotated 1/8th of a turn or 45 degrees and a hole plunged in with the 4mm cutter.

Then a small top hatted pin id pressed into the hole with the top hat 4.36mm in diameter.

Close up of the bits. The idea is you broach one slot, turn the bush so the pin fits into the previous slot and do the second. Wrinse and repeat untill all 8 are done and with a bit of luck and fair wind you get this.

The bit on the top is the top drive that carries a shear pin and this one is a spare I have and is pressed into service as a gauge.
So that sorts the bottom bit out.
Top pulley is splined identical and fits onto the splined shear top hat but to save all this messing I propost to fit the shear drive direct to the pulley.

Blank pulley, shear drive off the machine, spare shear drive and a gear.
Idea is to transpose a new plate onto the pulley boss, drill for a shear pin and bore and ream 5/8" like the original which was done but I forgot to get a picture of it.

So later on today we finish up with this. Pulleys done, belt fitted and a jockey made up of 3 spare sealed bearings to take the tension up.
Tested this at 1600 rpm which is as fast as this one goes and there is no noticeable difference in noise with the drive engaged or not.
So very well pleased with this and this drive is a clone of the finer ratios and not the one in the original picture.
The original has been replaced as tomorrow I have to thread some imperial threads onto metric shaft sizes for a Wadkin bandsaw at 7 tpi and 14 tpi then the belt drive will be re-fitted and hopefully stay on for a long while.