How to replace a very old gas fireplace?

I would call the local gas company and get their opinion. Because it's so old, I wouldn't detach it myself. There could be leaks or other problems that you can't foresee. You don't want to cause a gas leak when you remove it.You might also need someone to cap off the gas connection. Have you considered a pellet fireplace insert? That would be beautiful and fit into the firebox of your fireplace. Plus you'd have something you could use for heat without using gas or logs in your fireplace. Just a thought.