Fireplace lintel?

Hi

I've inherited a wood burner that is being installed in a couple of weeks.
I've opened up the fireplace in my Victorian house to the builder's opening, with the supporting arch above. (I will likely repoint the brickwork and clean the bricks up.)

The arch looks and feels in sound condition.
It begins at the start of the chimney gather.

I've included a couple of photos and have annotated one.
Red circle: start of gather- brick on left hand side is broken.
Yellow line: height of wood burner after accounting for height of hearth.
Green line: possible concrete lintel height? line is at the base?

I'd like to see what your views are on the following questions.

1. Would I be ok to leave the arch as it is, and not install a concrete lintel, or is it best practise that one is installed?

2. If I need to install a lintel, would it be acceptable to install at the green line (this would mean removing the bricks at the start of the gather circled in red) or would it be better to install it the brick course below?

Thanks for your help and guidance in advance.

depends what you want. One option is to clean up the arch and leave it exposed. if you want a square top to your opening, then you need a lintel to carry the 8 bricks that you will use. Again the height of where the lintel will be is a matter of taste.
Frank