You could try refinishing the screen using paint designed for projection systems. They come in several different colors and gloss levels, from white to medium-gray, and reportedly can deliver results comparable to professional Da-Lite fabric screens. The custom products like Screen Goo are very expensive (like $120 to cover a 4x6 screen) but reliable:
here it is at B&H Photo. Or there are DIY solutions where people have mixed their own formulas using home-store paints:
here's the Beginner's Guide from the AVSForum.