My advice would be to replace the plug correctly, but quick cheap fixes are available. I had the same problem years ago. Some plumbers sealant (Boss White) and a small self-tapping screw solved the problem for about 5 years until I got round to replacing the plug properly - the screw even made it easy to remove the old plug.
To be on the safe side I would replaced All the core plugs because if one is porous the others cannot be far behind.
You do not say what engine you have fitted as the 1275cc core plugs are quite different from the 948/1098cc ones.
The 1275cc ones are 33.5mm dia before fitting.
I would try and do a temporary seal from the outside, depending on where the pin hole is. I would not use any radiator sealant that you add to the coolant.
Previously (on my S1 Landy) I had a weeping core plug at the rear of the engine, which means an engine out repair,which I was not keen on at the time.
I managed to clean the outer surface of the plug and used a 2 part "steel epoxy" to completely cover the plug, as a temporary fix.
This was about 4 years ago and it is still sealed. Obviously drain the coolant and make sure the plug surface is clean & dry, oleo apply some rust converter on the outside to be safe.
Ufudu
1960 Morris Minor 4 door saloon
1957 Series 1 Land Rover