I'd be looking at replacement valves and if needs be re seaming it, if that fails then put O.E. Boxes it, after market exhausts are not one of my personal favourites, they're cheap and rarely last any where near a s long as the originals, either the brackets corrode or the packing gets blown out and becomes really raspy, not everyone's cup of tea I know because every one likes a loud exhaust, in my opinion the standard system cannot be beaten, maybe by an Akroprovic set up but That's been made using the best materials. If they've been on there for 15 years give or take I'd be reluctant to put anything on that's not going to fit exact and not going to last anywhere near as long.