The consensus on the GCD discussion lists and the Comic Books: The Golden Age Facebook page (26 December 2015) is that the Captain Marvel, Jr. figure is a Raboy paste-up and the rest of the art is not known for sure.
Michael Fraley form the Comic Books: The Golden Age Facebook page (26 December 2015) reports the back of the head is a swipe from an Alex Raymond circa 1941 Flash Gordon page.
Roger Hill confirmed on the GCD Chat discussion list that Gene MacDonald did no Master Comics covers (per an interview Roger did with him before his death) (26 December 2015). Original indexer listed Gene MacDonald ? under pencils and inks.