So you are saying the area above the door,the space that has the gap,was actually wood,which supported the bricks above?
Was there wood inside the frame of the home supporting the interior walls also? Separate from this outer lintel?
You can replace this opening with limestone facing.
My brick layer used a stainless steel bracket,it had an L-shaped base with hole to attach the unit to the wood wall/header,then there are different lengths of these, depending on the thickness of the stone to be installed.the end of the bracket turned 90 degrees and the mason would grind out a slot for the bracket to sit into the stone.
This method he used for at least 7 different window/door openings for the village hall here where I live, 9 years ago.None have failed.
I just ran across one of the brackets in the last few days,I can post a pic(if I can get it to load) and you can see what it is I am describing.*Note:The system will not allow for the close up image,too many kb?, can't see how,anyway, you can get these brackets or similar ones from any stone yard/brickyard,etc.
Hope this helps