I have a friend who has the RetroSound system in his 56 190 SL and he loves it. Long ago I bought a Custom Autosound AM/FM/Cassette (it's really a Kenwood unit with the case modified to fit the Mustang) and I'm very happy with it. It came with aux inputs so I can use my iPod. I don't think they make that particular model any longer but you could also look at their current offerings.
There are also people that remove the guts from the original radios and replace them with modern guts. I don't have any information about those but I've seen some reviews on the Vintage Mustang forum.