can you produce and sell a replacement device for a patented item?

If the part that you want to replace is not itself patented, then yes, you can make that part and sell it, even if the sonic toothbrush is patented. The patent on the toothbrush prevents someone from making, using, selling or offering to sell in the United States, or importing into the United States, that article during the term of the patent. I would not assume that anything is legal unless you search into the facts to see if the brush head itself is patented. You could call Philips and ask them for the patent number(s) for the toothbrush and brush heads.