You could make a silicone mold of the original lens, then cast it in a clear urethane resin. This will give you an exact copy of the original. The problem with this type of resin, though, is that there will probably be many tiny bubbles in the casting that were introduced during mixing. To get an optically clear, bubble-free piece you have to pressurize the mold to 60psi after pouring the resin. You should also have access to a vacuum chamber to pull the air bubbles out of the silicone before you pour your mold. Make sure that the silicone you use is platinum-cure RTV, not tin-cure. The tin-cure kind can inhibit the curing of the clear urethane and you will end up with a tacky surface.
smooth-on carries all these materials and has a how-to section on their website www.smooth-on.com
. "Crystal Clear" is their clear urethane. Make sure you get the right formula for the thickness of your lens. There are different ones for various part thicknesses.