i recently purchased a house that has a fireplace. it had a steel firebox which was connected to 4 vents in the brick mantle(I think its called a heatalator). apparently, the cap on top of the chimney was not there for awhile because the back of the steel box was completely rusted away allowing smoke through the vents into the house. i cut the steel out totally exposing all the bricks,including the vents.what is the best approach to fixing the fireplace?