As opposed to most of the fireplaces posted on this forum, your stone fireplace is the real thing. Too bad about the brick details and quarry tile at the bottom. If not for that, I'd just clean it and keep it.

Adding a mantel doesn't seem to be the next step in making the fireplace more modern. Mantels are more traditional.

Why not have it done in concrete you just leave the stone and cover in polished concrete BTW ledger stone is a dated look IMO

Hi Andy,
We offer a very modern Supercast Wood Beam Mantle that would look great above your fireplace. If you're interested you can check it out at our website: http://www.outdoorrooms.com//wp-content/uploads/2014/10/Supercast-Mantel-Sell-Sheet.pdf