I wouldn't use a scanner, I'd go the same route they use for oil paintings etc. Find a place that has a high end camera, and by high end I mean a 50k camera that can do 400-500 megapixel. Museums use them. You can find places that will do this depending on what city you're in. The colors will be more accurate. I've done this before for movie poster scans that needed to be restored and converted to LP covers. It will cost you a few hundred to get done but the results will be worth it if you plan to print the image at any point