I think your best course of action is first to identify 2 or 3 electric inserts that you think might work for you. The manufacturers of these unit will (should) specify the kinds and dimensions of fireplaces into which they can be inserted. They might have 800 customer service numbers so you can chat with them to sort things out. Your next step, and the most challenging, is to find a really capable installer who will put it in for you at a reasonable cost.