Is it possible to take a closer up picture of the core plug. Plus I stand to be corrected, but some on here are referring to it as a frost plug. These core plugs are fitted as they are needed to remove the water jacket casting mould when the engines are made. If you rely on them to let pressure go should the coolant freeze, there is a good chance the frost will bust the whole block. If the core plug has pushed out a little it is easy to simply measure across to get the size. A lot of engine shops carry a stock of various sizes, and they are not very expensive. Get the old one out, clean up the hole and get rid of all the crud. You can add a drop of a sealant of your choice, I used to used gasket sealant and find a piece of wood and gently tap in with a hammer. Or find a big hammer that fits the plug and use another hammer to tap the big hammer. It's a straight forward job once the battery terminal is off and the starter out of the way.