Upon getting my fireplace/chimney cleaned two weeks ago, I was told that I had a serious fire hazard inside my firebox. The "ceiling" of the heatilator shows extensive rust/corrosion to the point of major holes. (see pic attached).

I'm looking for my least expensive option for repair (but also safe, of course). Is it possible to just replace the ceiling portion inside the firebox? Or just the firebox itself without having to get an insert? If so, where/who/what? The fireplace and blower have functioned great (without knowing about the holes).

Side note: I did have this fireplace and chimney inspected upon purchasing the house two years ago. It passed with flying colors - "barely used" he said. Apparently, none of this was noticed by the chimney/fireplace expert. it seems close to impossible for this damage to occur in such a short period of time. He did seem aloof that day and also told me my fireplace didn't have a blower.