You can buy replacement freeze plugs made of some rubber type compound, with a flat washer and a bolt/nut. Use a cold chisel (screwdriver) to poke a hole in the old leaky one, and pry it out. Clean the hole as best you can, and push in the rubber one, tighten the nut. The rubber ones are just easy to put in, as you don't have to try to pound on something hidden behind a manifold or whatever.
tom