Welcome to the forum!!
If you are lucky the portlight will have the manufacturer stamped on the inside of the window, then with the dimensions research the model from the manufacturer's website. Otherwise you can do a little research and match up the model with dimensions and find the one you need on Lewmar, Beckson and SBO website
BeneteauOwners.com search . If you cannot find one, you can always have one custom manufactured stainless steel ones by Man Ship supplied from
www.marinedepotdirect.com (provide them the dimensions OR remove the portlight and send it to them to fabricate a duplicate. I have 4 port lights on a 1984 boat that does not have a manufacturer's stamp and the size does not match any commerically avalibale light so I plan to remove one and send it to them to fabricate 4 new ones.