I'd take out the first row of tiles from your hearth and use one of them to replace the flat tile. I'm not sure about the other one, you could take them all off and use an off white rounded tile to match the patterned tiles or you could just get a paint match and paint that bit in the right colour?? Or perhaps use a piece from under the mantle piece where it would be less noticeable? It would probably take you a few years to find an exact match reclaimed one as it's not a common colour.

We have a very similar fireplace, it's Art Deco style. My husband glued them together as we struggled to find a match. Not perfect but good enough we felt, so have a go!

The fitter can put a threshold round the tiles for the flooring to go up to. The tiles would obviously be lower than the rest of the floor, but if you can live with that then it's possible. Love to see a pic

Hi! It's hard to tell if the tiles are original. They might be original but they look potentially more early to mid 20th century to me. They are pretty though and in good condition. If you don't mind them then I'd tend to keep them. Even later alterations tell a story about the history of a house, so even if it's not original from first built there can be value in later additions, if you like them.