This is not a simple hiss removal job. I've got well over ten years experience with this kind of stuff, and specialize in restoring tricky analog audio.
No matter what, there are going to be digital artifacts left behind because the audio of the cats is so low in the mix. The question is how much of these artifacts can be removed without destroying the integrity of the audio? I have a mastering suite worth well over $10,000 and I have the tools for this kind of job..