Structural problems if I knock through my fireplace?

Hi everyone

I am thinking of restoring this fireplace to its original glory

this I mean knocking out the relatively new bricks under the stone lintel to open the fireplace to how it was, keep the plaster off the chimney breast and repoint the bricks with Lime mortar. Here's a better pic of the lintel. first point is structural integrity. The lintel doesnt sit on as much old brickwork as I'd be happy with.

A 2 pence piece is there for scale. The amount of lintel on the bricks is 7.5cm The right hand side (facing the fireplace)

left hand side. The amount of lintel on the bricks is 8cm finally there is an angled piece of stone that goes down and across the chimney breast. After pocking my head up the chimney, its obvious that this channels the smoke up the chimney to the right. Is this normal? I've never seen this before, and a bit concerned that the angled stone introduces a sideways force. Here is a piccy that shows the slanty stone help or advice is much appreciated.

Thanks

James