Best way to run 1st edition Forgotten Realms? (That isn't AD&D)

[S'mon]

If I wanted to run FR Grey Box with a non-D&D system I'd use Mini Six (based on West End Games' D6 System as in D6 Star Wars) Mini Six: Bare Bones Edition - AntiPaladin Games | DriveThruRPG.com - I ran a Primeval Thule campaign in Mini Six and it worked very well.