I guess that would be some kind of plexiglass, but if it took such damage from hydrocarbon solvents, it may be made of the more brittle styrene or vinyl based plastic. Given the extent of the damage, you may have several choices available with nothing much to lose.
I would not expect you would be able to source a direct replacement, but the good news is, provided it is not extremely thin, you can grind away with abrasives, buffs, etc, back to a new surface, and get it to go transparent. If the solvent damage goes into deep discolouration, you have the option of contriving your own. First step is to figure out how it is held in there, and how to separate it from it's bezel, and how to re-seal it when you are done.
In my past, I have taken apart oil sight glasses on some industrial vacuum machines, and I opted to replace with circular discs cut in 3mm polycarbonate sheet from a DIY store, sealed up with epoxy. I did rub one with 400 grit car body paper, with soap and water, followed by 800 grit, then I moved on to toothpaste and a little Dremel buff. It actually worked!. One thing to watch out for is that if the buff is too aggressive, it can end up pushing overheated plastic along. By far the best was the replace with new discs cut from sheet.
It was also pretty clear to me that I could have used real glass, but that stuff is hard to cut circular. Glass discs of various diameters could possibly be purchased, but I just took the line of least resistance. Not exactly a lazy approach, because it took a while, and quite a lot of care, but the polycarbonate sheet worked well. Any sort of plexiglass, acrylic, whatever should be OK against oil, and alcohols like IPA. Anything stronger, like acetone, xylene, or styrene monomers would likely wreck it - and I think that includes brake cleaner!