Will uncovering the air vents allow 100cm2 of air circulation?

My chimney flue is damaged and rather than getting an expensive to run electric fire, I've decided to opt for a flueless gas fire. They need 100cm2 air circulation in the room, which is a dorma attic on a terraced house which has been converted to the living room and I know that there are two air vents on the sides of the dorma. However they're covered up by wallpaper, and probably plasterboard underneath that. If I uncovered the vents would this provide the circulation needed, or do I need to have something new fitted? The gas fitter I spoke to recommended something called a black box as something totally new, but I'd like to avoid that if possible.