Joists protruding into constructional hearth

[LMCol]

Hi all, new here.
As part of my renovation I've uncovered the original fireplace opening in my 30's semi. I intend to reinstate a proper hearth, tile and get a HETAS engineer to take it from there and install a wood burner.

The concrete constructional hearth that was in place (below a 70's outset gas fire) is/was very cracked and crumbly. The hearth is supported on a brick perimeter which also supports the joists that run perpendicular. As you can see there's currently a fair bit of stick-out and I'm unsure what the current way forward... Do I:

A - Leave as-is, build a form around the opening and pour some fresh concrete. As far as I can tell, I'd then need to make sure I choose a stove which is compatible with a 12mm non-combustable depth

B - Trim the joists flush with the inside of the brick perimeter to minimise the combustible content

C - pull up the surrounding floorboards and build additional brick support piers for the joists, shim and then trim them so they are fully outside of the hearth.

One pic is the current state of the hearth, the other is right after the infill was removed but before the flooring was taken up

Thanks in advance

 

[Deluks]

B.

Most stoves these days say something like 'hearth minimum depth of 20mm' or something.
Unlikely you'll need a stove with over 5kw output.