Damn, that’s annoying. If and when you replace it, how are you going to keep the squirrels at bay? If they did it once they will do it again, no doubt.
There are cheap lenses for PIR devices out there on ebay, etc, but how you would match it exactly, and or fit it correctly, in a sealed way, if you did, I have no idea. Depends how handy you are.
If it had happened to me I would quite like to try, and would probably trim off the existing with a modelling knife, and do the same to the bottom of a replacement, and then seal it in place with clear sealant. Lets face it, you can’t make it worse than having a big hole in it.