Just a couple of thoughts - if any of this is actually crud stuck to the outside of the scanning assembly, perhaps try spraying some Lysol on a rag to wipe it down. Lysol tends to do a good job getting sticky stuff off.
If there is actually dust inside the scanning element, then it must not be completely sealed. Perhaps look for any openings and spray compressed air there.
If there are scratches, furniture polish like pledge/endust might give a very brief improvement.
The only other options I can think of are either complete disassembly of the scanning element (if that is even possible), or dunking the part in isopropyl alcohol. From my own limited experiences with scanners, those tend to end badly.