I recently scratched up the lens on my GoPro to the point where I was getting some serious lens artifacts when the camera was pointed at any source of light. Contacted GoPro themselves and the best they could do was offer me a discount on a new one.
Then I discovered that there are third-party parts manufacturers that are selling lenses for the GoPro. These moderations are popular among the RC Aircraft crowd which is a perfect coincidence as I believe I got the scratches on my lens from zipping it about in a quadcopter.
Here are two tutorials: One installing a Sunex Lens and the other from Ragecams: