Thanks for your replies Chaps!

Arnold.
The displacer feels surprisingly light, for it's size. About the same as Boggy's Robinson replacement.
I thought about hollowing the centre, making rings. But, wasn't sure what the heat might do to any glue used. I think unglued solid discs might be best.... For now.
Nick.
Thanks for your usual enthusiasm!
I have a piece of brass tube, which should clean up at 17.5mm bore. Piston will be brass too, probably.
I am hoping it will run on a small, single wick meths burner. If it runs on only a tealight, I'll be delighted!
I have a Contax LTD engine..... Running this past 14 months, behind my computer. That is my inspiration......
Thanks for the link. Read some time ago...... Always good for a refresher!
Managed a little progress today.
Attached the disc to the 4mm shaft, using a 3mm countersink screw, and my faithful high temp sleeve lock.......

Turned the o/d to bore size........

Drops in, so far..........

A gentle push, gets it to the bottom.......

This then locates the top deck. Holding it level, and concentric with the displacer shaft.......

I think 4 off, 3mm screws through the side, into the deck will locate. Allowing future dismantling and accurate re assembly.
After an hour or so in the oven, to dry out any remaining moisture, I will reduce the displacer to .1" clearance.
I'm expecting
no progress during the next few days..... A little Crawling Person will be demanding Grandad's attention!

David D