Legal issues for remakes

This thread is rather interesting as I'm about to release RetroSphere a game based upon te awesome C64 Trailblazer. I have tried to track down the author (Stuart Southern) but have run out of options. I know he authorized the original ROMS to be distributed as long as no charge was made but as for a remake I don't know.

There is no media used in my game from the original its just the basic concept thats been borrowed.

I have also credited him for the original idea in my game.

So am I in dnager of being sued? Who knows...

Any help would be greatly appreciated...

Thanks.