Many of the Fawcett comics appear to be public domain, but some are not...you just have to be careful you don't use elements that originally appeared in the copyrighted issues. Read the issues to distinguish the difference. Fawcett characters that appear to be in the public domain include:
Bullet Man
Bullet Girl
Captain Marvel
Captain Marvel Jr.
Freckles Marvel
Ibis, The Invincible
Lt. Marvels
Mary Marvel
Minute Man
Shazam, The Wizard
Spy-Smasher
Uncle Marvel
Captain Nippo
Dr. Sivana
IBAC
Jeepers
Macro, The Giant
Magnificus
Mr. Banjo
Mr. Mind
Nippon
Sabbac
Characters that are NOT public domain:
Beautia Sivana, Black Adam, Tawky Tawny, any reference to Sivana living on Venus or Captain Marvel traveling there....I don't think the name Thadeus Sivana came from a public domain source either, but still looking...
Anyone else care to add to the list?