The only other option is gas logs. Vent free White mountain hearth makes some very realistic logs

I'm in a similar quandry. I'm wondering what you ended up doing?

Nothing, sadly. Have yet to find a solution beyond tearing it out and putting it all back together.

I have the Valor historical gas inserts and they look great! However, they were a squeeze for our box and slate mantel and yours is certainly smaller.

There are some special cast iron inserts available for tiny boxes but mostly through British companies. I listed some great websites here:

Victorian Fireplaces

British Heritage

Gas Coals

Antique Fireplace Company

Keep us updated!!

For tile, look at www.lesperancetileworks.com

Your 8-1/2" depth is a problem for any gas insert. What WILL fit is an open gas coal fire like the Victor gas coal burner, which would allow you to keep your original cast iron surround and summer cover. Victor Gas Coal Burner

I know this is an old thread, but I have a similar situation in my circa 1900 house and I am looking at the Napoleon Torch Gas Fireplace. Napoleon sells it as an ultra modern fireplace, but I'm thinking that it might serve an older generation of house if the front panel could be changed slightly.
Napoleon Torch

I would also recommend gas logs as an option. That is quite a project you have on your hands.

I don't know whether the OP is still around, but I am now looking at fireplaces made by the Archgard Fireplace Products Company, specifically the Brigantia Eloquence. It is 6 inches in depth.

OP here -still following this and looking into the able as well as the Victor Coal gas burner mentioned above. SO many good ideas!!!

It is quite likely that the Windsor Arch front on a Valor Portrait insert will fit, but not inside the original casting. The original casting gets removed and the Arch then takes the place of it. What's missing here is accurate dimensions and someone who knows how this is done. Portraits have been used hundreds if not thousands of time in situations like this.

HHS - what measurements are needed? and, yes, what is missing is someone who knows what I can do here. That was what I was asking - what could be done. I'm happy to replace the existing arch and summer cover, but the marble mantel is not negotiable.

Marble mantle shown above with the Valor Windsor Arch front and polished highlights. The dimensions needed are the overall width and height of the existing cast arch, and the dimensions of the arch cut out in the marble behind the cast arch.

OK! i will measure both of those this weekend!

Opening within the cast marble as shown: 32.875” (subtracting .5” for medallion hanging down/if insert can’t tuck behind it would make it 32.375”) high x 29.75” wide

Opening within the inner marble piece: 29.75” h x 24.125” w

Unfortunately it looks as though your mantle is just too small to accommodate the Windsor Arch Front as it is 31" wide by 34" high

http://www.coalbaskets.com/coal%20baskets%20vent%20free.htm

I have a similar situation with a 100 year old house that has 2 coal burner fireplaces. I found an insert I think will fit the small dimension of ours. Here's a link to the one we're considering. https://www.victorianfireplaceshop.com/PRESIDENT-Small-Gas-Insert_p_2545.html