Have looked on this site. It is based on the Isle of Man.
I suspect that the pistons at the top of the pageare for the Douglas Vespa 2 stroke scooter. I cannot remember the bore & stroke of a Douglas but a figure of 60.8 x 60 comes to mind in that equation. These pistons are for a lesser size. Unfortunately some traders (especially a well known website) calls all Vespa related machines & products to be of Douglas name. At the bottom of the page there is a couple of pistions for later Mk 5s. Bit pricey though.
I have heard that some owners have fitted Honda pistons.
Please also note that Mk One heads & pistons may be different to those of the later models, ie Mk 3 sports onwards, including mk 4, 5 & plus models.