Burning oil, you want a new chamber of insulating firebrick.
You should get a modern burner as well which will be far more efficient and cleaner. With a modern burner you could probably go with a precast chamber and backfill with vermiculite (mica pellets).
The burner is sitting in the ash pit, so all those heavy firebrick are of no use. the chamber wants to be insulated so it is not heating the ash pit. All those deteriorated bricks are just so much rubble and don't mean much. The existing chamber is still in one piece.