fireplace cover

Hi, Can anyone help with how to fill in this hole? It used to be a gas fireplace but it's been removed. The flue has been taken down below roofline and tiled over. What's the best way we can fill and plaster so it's flush with the rest of the wall? And do we need an air vent seeing as the...

I'm only concerned with covering them nearest to the wall because obviously I can't cover the stove. There are high up on the left side and then low down on the right. It's a really old and wide chimney that arches at the top so I'm worried brick work would look weird and also come out too far...