Fireplace Insert for Rusted Firebox?

I had my fireplace inspected the other day and was told that the firebox is completely shot (rusted from never having a cap) and that I should get a hearth stove and chimney liner. The room and hearth are small, and I would much rather have a wood burning fireplace insert but was told that they require a fully functional firebox. Is this true? Are there any that don't?

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