I repaired many of those with good results, as Dacor was able to provide all the components way back when. That lens should unscrew, but it has a narrow serrated edge and will probably take a little ingenuity to get some tool to grip and unscrew. Take the boot off, remove the inlet fitting and lens, and completely remove gauge component from the plastic case asap for a good clean up with fresh water.
Look closely at all areas of the plastic case for hairline cracks, especially where the hose attachment fitting is. Also around the outer rim where lens screws into the case. I probably still have some cases and lenses for those. I think I even have a couple of complete gauges like that. I always liked them over the USD gauges since they were repairable.
I'll look for that stuff today in hopes of selling you some parts at an exorbitant price. Oops, it's cold and rainy today.... have to charge extra for a trip down to the barn.