I was wondering about the material or if it is a scratch not a crack, glass does not start a crack in the middle. Cracks in class always start at an edge or keep running to one edge if sufficient force is applied in the middle, plastic can crack from the middle. If you are able to get the cover off buying acrylic or polycarbonate as a cut circle is easy anywhere these days, but make sure the dedgeds are flame polished it takes out the stresses.I redid one of the lenses on mine as it cracked the same as yours. It's a giant pain in the butt to replace as it's plastic and not real glass. The plastic is bonded to the body of the gauge and I had to grind the entire lens out with a dremel. There have been a couple people over the years on the forum with this issue if you search it. I unfortunately don't have any pictures of mine when they where apart but can say it's not a fun job and hope my other side never cracks. I had picked up a couple of acrylic disks on ebay to use as the new lens. The 85 x 3mm ones fit fairly well. Transparent 3mm thick Mirror Acrylic round Disc Diámetro:85mm T7F5 | eBay