Hi all, could use some input on this because I'm a bit of a novice and need to make sure I'm using the right tools for the job!

Our house was built in the 1940s and we've lived there for almost 30 years, and little did we know there was a badly filled-in air brick inside my build-in clothes wardrobe. In recent years I wondered why I had such a bad clothes moth problem and why my clothes smelled fusty.

The air brick is 6 ft above the floorboards, directly inside my enclosed wardrobe. It is in a contained space consisting of only my clothes wardrobe and bookcase inside my bedroom. Photos here. https://imgur.com/a/KCKaehO

It seems the previous people did a bodge job and filled it in from the inside using only plaster filler, so of course over the years the plaster has deteriorated and crumbled - and has now had an explosion of mould growth that spread all over the walls and ceiling in the upper storage section of my wardrobe. Had to absolutely blitz it with bleach and throw a load of stuff away. (and for anyone wondering how I didn't notice for so long - I use the upper level for storage so I couldn't see what was happening until it was too late)

So needless to say this air brick needs filling in because of its ridiculous location. Now the question is what do I use to seal this flipping thing up for good so that this doesn't happen again?

It would have to be sealed from the inside as this is an upstairs room. Would it be feasible to scape off all the old stuff, seal it up with repair cement, then re-plaster and paint? Any help and tips would be greatly appreciated. Brands and recommended products especially. Thanks!