Oo-er, that doesn't look good, does it? You could bodge it with some de-rusting treatment and some filler, but I don't think it would last very long, maybe a year or two, before the rust came back.
You could remove the wing, get it stripped and AP coated and get someone to weld in some repair patches, and that might be an attractive option depending on how much work you could do yourself and how much you'd be paying in labour charges. Getting rear wings off can be a pig of a job, people normally just grind off the fixing bolts and refit them with stainless screws into the wood from inside the wheelarch - otherwise you have to strip out all the rear cockpit lining and the rear section of the tub to get to the bolt heads.
If you just want to keep the car looking smart and useable for minimum outlay, you could go for a pair of fibreglass replacements. That's a standard solid colour, so any car repair shop should be able to do a decent match with the paint.
Best of luck whatever you decide.
