Well, supposing the orange rubber is in good shape, I'd imagine you have to remove the grating, I'd recommend the inside rather than outside. Since this is all molded in, it is going to take a lot of time to cut each prong out of the rubber. Once you get the grate out, remove the paper element. Using another similar filter medium, cut to fit, replace it all, and put the grate back in. Viola, restored. Of course, you may want to scrub and santize while it is all apart so it appears new.
Simple as that!
