Thank you for the photos. It does indeed appear that an insert was used in the fireplace at one time. The fireplace can certainly be converted back to a standard wood burning application by removing the liner and installing a new damper assembly. I do not recommend using liner panels, as they do not work well in a masonry application. Instead, the interior of the firebox can be renewed by using the Rutland Castable Refractory Fireplace Cement - 12.5 Pound Tub
. A layer of this cement can be parged over the worn firebrick and will restore a level surface. The cement will also be able to withstand impacts from shifting logs, while panels would immediately crack.