I don't know how comfortable you are with disassembling units like that, but it is the quickest way. I got a free hand held scanner that smelled really bad like cigar smoke, so I took the whole thing completely apart and put the pieces in about an 18qt tub with 1/2 cup dish detergent and hot water. I let them soak for about an hour and then began gently brushing everything with a toothbrush. I did both sides of everything and ran them under warm water when I was done. From there, all the parts were placed in an open (very shallow) cardboard box to dry. Usually I would put it outside in full sun and occasionally turn the parts over to make sure each side got a few days of full sun. This took about a week.
When I was confident everything was completely dry, I reassembled the scanner and applied power. Not only did it work just fine, but the cigar smell was completely gone and was replaced with the strawberry smell of the dish detergent. I certainly cant complain about a free scanner that only needs a bit of cleaning. I'm not sure if I would pay much more than a few dollars for something that needs that kind of work put into it, it is time consuming (time is money), and there are plenty of other scanners for sale pretty cheap that need nothing. Only buy something like that if it is heavily discounted, even if it is working fine because it still needs work.