Crack in fireplace lintel

I've removed a 60s tiled fire surround and hearth (also the plaster from the walls) I've found a crack in the lintel over the fireplace. Basically I'd like to know how much of a problem this is - it has probably been there for decades. Photos attached.

The situation is 1920s semi, solid wall (no cavity) construction. Therefore on the interior of this outside wall I am fitting Gyproc Thermaline insulated plasterboard to improve thermals. The original plaster was coming away from the wall in places anyway, almost certainly due to a previous damp problem (recent new roof and flashing when I bought). Chimney breast is on the outside of the house.

My options are:
1) open up the fire chamber and have either gas or multi-fuel stove fitted;

2) have standard real-effect gas fire put in (not overly keen on these);

or 3) open up chamber and put a fake basket in for appearance only, put fireboard incl. flue vent above so drafts and rubbish don't come down into the room. This on the basis that the improved insulation will mean a fire isn't necessary so much for heat as for appearance. Chunky radiator in the room and I'm having the bay window double glazed. It may be ok on heat... don't know until I've sat in there for a winter!

Do I need to have the lintel replaced or will steel reinforcement bar mean it is still good enough? (if it needs doing so be it, just that I'm being nagged to get on with the insulated plasterboard so it feels like a room again

The lintel is not going anywhere. Ignore the crack

Thanks Woody, very helpful.

The way the "aesthetic direction" (that's what she calls it) is going, I'm going to be removing those bricks forming a smaller fire back/surround to open the chamber to what I assume to be original squarish shape. Some of them are a bit loose anyway so I hope they don't have any supporting function!

I guess thinking about it the ends of the lintel are jammed in place, set into the main brickwork and they can't spread out, so as you say unlikely to go anywhere.

Sounds good. Unless of course the aesthetic direction suggests we need a taller chamber..... (ssshhh, nobody said that or even thought it).