Saving the fire place.

Was very pleased with the fireplace when we moved in here. But dismayed when we tried to use it. Turned out the flu damper was mortared shut. From the outside. By cutting some access holes through what would be the outside chimney surround I found there was a chimney there at one time. And found the mortar nicely slathered over where the smoke box should be. Apparently there was never any use of fire brick in the past. At one time it burned natural wood, but now has gas logs. We have yet to use them ourselves. No venting of gas logs just seems like not a good idea to me. (This may be a long post...) I want to line the fire place with fire brick, of coarse, but need to (re)construct the smoke box and chimney. Where can I dig up dimensions for a smoke box that's appropriately sized for this rather large and nice looking fire place? Chimney building will be the next level of concern.