If it does not look good now it never will unfortunately as there is really nothing you can do to an existing surround to make it look better other than clean it and an opaque shower curtain.

Take it out and create a beautiful tiled shower with a million grout lines to clean. :D

Don't hate too much on the prefab. It ain't pretty, but they're sooo easy to clean and just darn practical. As long as it's in decent shape, just love it for what it is. And a pretty shower curtain goes a long way!