I'd call in a mason for advice and to make sure the crumbling doesn't cause any fire hazard.

Agreed. A brick mason can remove the crumbling bricks and repair the mortar. I used one to repair several bricks and the keystone over the door of my 1949 cape cod.

As for the hearth, a granite remnant, piece of marble, or slate slab are all good possibilities.

Agree with the others about the mason. Soapstone is a good hearth, too. Depends what you like. Can do many different surfaces. Since the fireplace doesn’t sit on the floor, you may be able to extend the current flooring. Check local code, though.

Yep, looks like you need to call a mason on this one.