www.FordRacingParts.com does not list a 514 stroker, but they do list two 521 strokers.
Both engines use M-6049-SCJB heads. These heads use a Motorcraft AGSP spark plug.
Pick the heat range that matchs your application, cross referance that to an Autolite (such as a 3926), and go to Autozone and get what you need.