As a home repair specialist, I know of no good way to repair a damaged track. Your idea of laying another track over the existing one will not work because of the extrusions on the bottom of the track and they are not designed to stack like paper plates. As far as replacement goes, it not difficult at all and the fact that the concrete is around it should not make any difference. Replacement tracks are not easy to find, due the wide array of manufacturers and the various design of rollers. While I know it's not what you want to hear, if you can't live with it, you'll most likely need to replace the whole unit, doors and all. Total job for labor and materials to replace standard aluminum framed sliding glass doors should be around $500.