me and my air brick

Hallo,
We've got an approximately 70-year old house with an air brick in the upstairs bedroom just below the level of the roof, below the fascia. There was once an open fire in the room but the fireplace has been blocked off. Currently, there is some sort of wooden box type contraption shoved into the air brick hole on the inside. The air brick causes a lot of cold and damp to the inner plaster around it. I am planning to remove the wooden contraption thing and seal this awful air brick up with four bricks and some mortar (the wall is two bricks thick), seal up any other holes in other bricks surrounding the dreadful air brick with mortar, and then, after knocking all the powdered plaster off, re-plaster either using plasterboard onto the laths, followed by one coat plaster mixed with PVA, or just one coat plaster with PVA, depending on the thickness of the holes. I am not a builder, but I did manage to repair a ceiling, floor and wall in another room with great outcome.
My question is about this airbrick: Can I seal it from the inside with bricks and mortar, then plaster, and later render over the airbrick from the outside? Also, am I correct in my assumption that the airbrick was there because of a previous fireplace?
Any help and tips would be appreciated. I am a true novice. I haven't removed the wooden contraption yet in case the bricks above it cave in.
Thanks

My old house was 70year old and the air brick was plastered over. I had to knock the plaster off and no nailed a plastic grille over because I kept geting damp in the corner of my bedroom. It solved my problem.
If I was you, I'd pull off the cardboard cover, cement in a clay airbrick on the inner wall plaster over and fit a plastic grille..

Thanks for your reply. There seems to be some mysticism and magic surrounding airbricks. I just don't see the function of these airbricks in modern houses, except to necessarily ventilate cellars or blocked up fireplaces. I know my house isn't modern, but it does have central heating and double glazing and no active fireplaces. All these airbricks seem to do is let in cold and bus noises. Why bother insulating when you have a 1 foot hole all the way through a wall to the outside? Nobody will sleep in the room and it'll be even noisier with just a small, thin plastic grill. It really is noisy, the wallpaper always peels off in that bit, and I'm such a fan of concrete and bricks. I don't think the wall has a cavity, but I could poke about in it once I've removed the wooden thing. I think maybe the cold and damp gets in if it is not sealed up properly. So just plaster or a bit of wood isn't good enough, which is why it either requires less (a plastic venting grill) or more (bricks and mortar). Paradoxically, it would probably be more difficult for me to put a plastic grill in than some bricks! But I'll think about what you've suggested. Thanks again.