Two products you might look at are Fiberon and Trex. A number of websites will come up using any search engine.
My husband and I recently rebuilt our deck using plastic composite decking material. We did a lot of research, and I've never seen one that's specifically designed to go over old wooden decking--although there's no cosmetic reason not to do that. That said, there are two things to be careful of:
1. Drainage. An 1/8 inch gap between decking boards is standard. If you lay the new floor over the old, will it drain properly?
2. Weight. The products mentioned above weigh about twice as much as wood. So you will now have three times the normal weight on the joists, posts and footings holding up your deck.