For those of you who've already read SSRat, SSRat's Revenge, Star Smashers, MRMR and the Deathworlds, try Rebel in Time. I was lucky enough to read this in a version with a blank cover, since most versions can't resist giving the plot away. It's also notable for having that rarest of things, an explicitly black golden age sci-fi hero.

For that matter, they're still pretty rare.