I'll second the Novus products. If you can't get your hands on those, auto parts stores stock polishes to restore yellowed/hazy headlights that work quite well.
AS for replacing, the frames on the grampian are removable, and new plexi can be inserted. Personally, I will be replacing mine with 3/8" surface mounted ports, a little thicker than the flimsy stock ones.
The stock grey gaskets can be found here:
Holland Marine Products