Is the corroded hole now irregular? The frost plug goes in ans stays with enough tension but leaks.around the edges.. or the hole is now over size so the frost plug goes in way to easy. Not enough tension to hold it.
I have fixed cylinder ehads with liquid glass. (its like muffler cement only thicker) yo get to at CTC or home hardware. Tech Steel
A good epoxy will work too but the rust must be removed from the cast iron.
Just epoxy the frost plug in once the hole is cleaned up. NLS Products - Red Hot 2000