Is a plug with a repaired ground prong safe?
No, that's not safe, Epoxy will not make a strong conductive bond to the ground wire in the plug. While the replacement prong may good contact in the outlet, you also need to be sure that it will make good contact to the ground wire in the plug. A simple multimeter test may show that it's connected right now, but the wire and/or replacement prong may shift over time, making the extension cord unsafe due to the lack of a ground connection.

Instead, replace the whole plug with a replacement plug, such as this Heavy-Duty 3-Wire Replacement Male Electrical Plug (there are many other varieties or replacement plugs available):