It sounds very much like cement board, aka wonder board or durock. It's usually used as an underlayment for tile floors, but it's not limited to that. I think a layer of cement board, covered with some kind of fireproof tile or brick veneer might be your answer. Ask a contractor or building inspector, or go to a local fireplace shop and pick their brains. Also, be aware that the building code restrictions on fireplaces in mobile homes tend to be stricter than the ones for stick-built homes.

I'm sure the fireplace is a prefab unit and they are rated for manufactured homes. See if you can find the brand of the unit and go online to find their website and see if they have replacement parts.