Matthew Modine and Richard Griffiths are set to topline the romantic comedy “Opa,” for Cinema Seven and “My Son the Fanatic” helmer Udayan Prasad.Pic — which follows the story of an American archeologist (Modine) who arrives in Greece to fulfill his late father’s work of finding the Chalice of St. John — lenses this month in Greece. Griffiths plays a colleague of the archeologist’s father. “Opa” is a co-production between U.K.-based Cinema Seven Prods. and Greek NIK-S Movies. George Pappas and Thierry Cagianut are producing for Cinema Seven, and Alexis Varouxakis and Nikos Sekeris for NIK-S. Cinema Seven is also in post on the teen slasher pic “Hollywood Horror.” International sales on “Opa” are being handled by Bristol Media.