Hi, don’t know if he has your replacement pistons but this bloke carries a lot of new / NOS / used Brembo parts:
https://www.ducatipaddy.com/index.html
HTH
Davros
Had a look at the Paddy Ducati site but pistons for the reproduction P05 calipers are out of stock with no indication of when they might become available as it appears that they have been discontinued.
I wonder if there is any mileage in the MGC GB getting a batch made. Surely there must be a market not only from the Guzzi fraternity but also from owners of all the other Italian bikes that originally used Brembos ?
https://www.ebay.co.uk/itm/Brembo-120267920-Piston-Kit-32MM-Pair-For-Brembo-caliper-F05-P105-P32A/303573901311?epid=687258214&hash=item46ae6a1fff:g:FbsAAOSwrHNexhfy
Not extremely attractive prices, but thing is available - sometimes. Sometimes it is worth to buy rear calliper from barely known bike, as they share same piston even if casting is “gold” series. I mean, Cagiva Mito/River.
Not sure about middle link above, but giovannimoto will happily answer your questions:)
If you have a tame engineering shop, they could fairly easily make you some new pistons up in stainless.