Well, you cannot feature NEW artwork on the cover, but anything that was from a public domain comic is fair game.
Fawcett is a bit trickier than the other publishers in that they did renew some of the comics, just not all of them and there didn't seem to be much rhyme or reason as to what they renewed and what they didn't. And, they continued as a paperback publisher before getting sold (briefly merging with the remains of the Popular aka Standard/Nedor/Better), divided up and resold . Because of that, researching Fawcett alone could take a day by itself in the card catalogs at the Library of Congress.