Airfix missed the air conditioning packs on the lower fuselage. Look for them just
below the wing leading edge. You can make copies from the ones in the Revell Beluga, or you can laminate some plastic card together. Whichever way you do it, it's a fairly easy fix.
Another easy fix is the leading edge fence. Simply cut the leading edge with your razor saw to the depth of the slat line on the upper surface and glue in a small rectangle of plastic card. Once it's dry, trim and sand the fence to its proper shape. The fence may be seen
here just outboard of the second flap track fairing. Also note the shape and position of the air conditioning packs. Here's the fence from
the side showing that it does not extend past the trailing edge of the slat.
This image shows the fence and some typical weathering of the upper wing surface.