I took the quill apart further to find out what needs doing. The original Jacobs drill chuck taper is not ideal as I cannot easily convert this to a collet spindle. An ER16 collet spindle is available with a 16mm shank; This is ideal for the bottom bearing as the shaft is 16mm, but it will be hard to sleeve or connect onto the original spindle.
The spindle simply has a ball bearing top and bottom with a few circlips. It’s certainly nothing fancy – with a bit of jiggery-pokery I should be able to get an ER16 collet system on there.
Of course, the first thing to do is to try it with the drill chuck on. We’ll see if that can at least cope with the loadings of milling. It’ll only get better with a good collet system.