Fireplace Ceramic Insert Repair

Fireplace manufacturer is out of business so I am not sure where to turn to fully replace the inserts and have spent a while on the web searching. I am fine with repairing if possible but I am not familiar with these. Weak spot is about 6” x 3” and has a hole about 2” x 2”. Any advice or pointers appreciated. Original manufacturer Temtex out of Tennessee. Model#TEC 36-5.

I just went to a local fireplace dealer here in Tyler and the sold me one larger than my current box. It don't matter the brand of fireplace. Then I cut it with a masonry blade in my 4 1/2' angle grinder to fit.