I read this children's book in 1964 or 1965.
In the novel in question, one or two young boys find a metal disc that he or they hammer on to the nose of a backyard spaceship he or they built, giving it the ability to exit Earth's atmosphere. HeHe or they are accompanied into space by an extraterrestrial being, who lost the metal disc on Earth. AsAs best as I can recall the alien being was from the (fictional) planet Eyopee"Eyopee. Strike any chords with anyone?"