I'd try to see if I can get a good pic of it maybe with something black in the background, or maybe take out the window and cut that section and put it in a frame, but that might be going too far if there are already other items of sentimental value related to that pet, that I kept. I probably would not go out of my way to wash it, I never really wash car windows anyway, other than maybe the windshield so I'd probably leave it there until it eventually fades away.
The best memory of a pet is pics and videos though. Take a lot while they're well and alive. I can't imagine how hard it is to deal with losing a pet.

I lost my other cat in the sense that she was let outside and got lost, and I miss her very much, but I think getting home to seeing it dead is probably much harder to take emotionally. At least when my cat was lost I knew she was probably alive somewhere, though it always bothered me because I did not know for sure, and still don't.