I have listed a set of Cosworth pistons on Ebay. They have a 66mm barrel and will fit the BS1 series engines and others.
The ebay listing number is: 223578622727
I used to vintage race an Arnolt Bristol, the pistons never made it into my engine at the time.
Admin Edit: Here's a direct link
https://www.ebay.com/itm/Cosworth-pi...kAAOSwbsNdIhQb