Re the lintel.. You could easily take a few bricks out above the old lintel and replace. If you’re worried about it falling down you could always hire a srongboy to support the brickwork. Did the same myself before christmas and it was dead easy.
But… If you’re plastering anyway and the lintel is sticking 10mm beyond the brick, cant you just skim the existing lintel with a very thin layer of plaster to build up to the level of plaster you’ll put on the brickwork.
Also.. maybe you’re putting a surround. if so, wont that hide the lintel?
One tip.. Remember the plaster will get VERY hot. It WILL blow and fall off or crack if you dont leave a decent space from the stove.
Good luck.
Some pics of my effort Here
Edit: just read it’s an inch not 10mm (god knows where I got 10mm from)