I've experienced two cylinder head separations on Beavers on Amphib floats. On the second occasion the sickly, sweet smell of AVGAS was very apparent in the cabin until the throttle was retarded to barometric manifold pressure. The windows were closed on both occasions - however in the Beavers that I flew you got wet in rain even with them closed.
So, yes, I do beleive that an exhaust leak could make it's way into the cabin. After reading the report I wonder how elevated my CO levels got at times - the distinctive chugging sound of a leaky exhaust gasket resulted in numerous trips to the hangar.