Firstly let me say that I have never tried this myself but I have heard that one way to bring up old rubber to a useable state is to boil it in radiator coolant. I'm told if you simmer it in the coolant (undiluted) overnight, it can make the rubber more maleable and flexible. Stinks out the place though apparently.

I pulled apart my airbox / carb rubbers etc recently. I used 3M Weatherstrip Adhesive to keep the joins all airtight. It works really well - makes a much better and longer lasting seal than silicone. Permatex makes a similar product too.