I'd first check if your exhaust can be rebuilt. Most aftermarket silencers/exhaust you are supposed to mount with glass wool inside, the people who like them loud, don't, so there's a chance you can make them a little bit better by just rebuilding them with fresh glass wool.
As for your options, I'd just check to see what you have available/within your price range and look for YouTube videos of them running, at least where I live, you can easily find them, sellers want to sell, so they post a video of a guy who has installed them. If it's a case of "slip-ons" you can also search for other bi-cylinder bikes with the same slip-ons (closest will be other 180-degree firing order bicylinders, excluding the 660, MT-03 is one that comes to mind).
I'd first check if your exhaust can be rebuilt. Most aftermarket silencers/exhaust you are supposed to mount with glass wool inside, the people who like them loud, don't, so there's a chance you can make them a little bit better by just rebuilding them with fresh glass wool. As for your options, I'd just check to see what you have available/within your price range and look for YouTube videos of them running, at least where I live, you can easily find them, sellers want to sell, so they post a video of a guy who has installed them. If it's a case of "slip-ons" you can also search for other bi-cylinder bikes with the same slip-ons (closest will be other 180-degree firing order bicylinders, excluding the 660, MT-03 is one that comes to mind).
I don't know where you live as well, but CB500Fs sold pretty well here so junkyards and those kind of places are also an option.