If the chimney is in a very bad way, you might well find it's cheaper to remove it and fit a suitable metal replacement which will outperform a masonry one. It will also need to be lined even if you don't replace it, liners are expensive. Do not use it as it is, it could be leaking deadly carbon monoxide into the house.
In my last two houses, I removed all the brick chimneys and put in metal, also gaining a lot of space.