[snowcone]

Apparently the modification was not done properly and there was a small gap between where the exhaust met the engine allowing deadly amounts of exhaust fumes and carbon monoxide out.

This might as well be written in Klingon.
"Where the exhaust met the engine"? Manifold? Headers? Engine pipe?
Still none of this is close to the actual cat which is further back.
If the exhaust was leaking at the previous position of the cat I would think the bonnet vents would have nothing to do with it at all and that the exhaust would have to be entering the cabin from the underfloor area. In theory it would not seem possible for exhaust gases to move toward the front of the vehicle and re-enter the engine bay against the positive cooling fan pressure that is forcing the gases towards the back of the vehicle and then pop themselves up out of the bonnet and then somehow make another directional turn and entire the fresh air plenum.
A bit like the bullet that changed direction several times and supposedly killed President Kennedy