get the rotted plug out before it lets loose and over-heats and ruins the motor. Then clean the hole and use an expandable rubber plug or use a block heater plug if there is room inside the water jacket for the heater probe. That would be the easy way. Pull the engine, replace them all and flush the old crap out of the cooling system, fill with good antifreeze. that would be the correct way to do it, but not the easy way.