1923 house with wood fireplace converted to gas problem
Any type of photo or even a rough drawing might help get more suggestions from the community. As far as the opening at the hearth being to large you will need firebrick to enclose it to the size required. In place of common cement mortar a refractory-type is used to bond the bricks together. It would be wise to dry- stack the bricks in the proposed location and see how it affects the draw from the hearth. That way you are able to add or remove bricks to control air draw before they become permanent.  Also, unless the chimney was upgraded at the time the fireplace was changed from wood to gas fuel, you should make sure the flue is the correct type. I seem to recall that the smoke from burning natural gas may cause an accelerated deterioration of the original brick and mortar chimney. It would be worth having it checked and looked at.