Depends upon choice of piston. If the current bore is 66 mm, you may as well go 70+ mm IMHO
If you are happy to be stuck with 'dedicated' pistons limited to those specifically for a Lambretta, then perceived wisdom from other Forum users would probably narrow the decision down to a replate of sorts. However, I have taken the 'risk' of utilising a DT250 piston in 70 mm TS1 standard plated bore with great success. Perhaps I have been lucky in ignoring the rules concerning graded pistons but I'd probably do so again
Personally, my choice goes with anything other than being limited by 'Lambretta specific' pistons based upon experience & cost.
Therefore, I would start the selection process by browsing t'internet for set-ups based upon alternative pistons as well as Grampian Motors, Wiseco websites etc