My brother buys a zinc alloy of some sort from Mondello and melts it and pours it into the opening on his oldsmobile heads. It's expensive to buy, but pretty easy to do.

He tried doing it by melting some old pistons down, but it didn't work very well. You need a tremendous amount of heat to liquify aluminum and keep it in that state long enough to pour it into the holes. It ended up all lumpy and parts would crumble off if you picked at it so he dug it back out and started over.