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. He or they are accompanied into space by an extraterrestrial being who lost the metal disc on Earth. As best as I can recall the alien being was from the (fictional) planet "Eyopee."
Could this be Rusty's Space Ship (1957) by Evelyn Sibley Lampman?
There was no way for Rusty Adams to know that the shiny metal disc he had found at the city dump and used on his play space ship in the garage, was actually the flying saucer of the Mighty Gwump of Eopee in Adromeda Galaxy. When tiny Tiphia, Gwump's messenger, arrived to claim it, Rusty and his playmate Susan Northrup found themselves taking an unexpected trip to all of the planets, except Neptune and Plato. Rusty and Susan save Tiphia from attack by ants and other hazards as he vainly tries to find the moon, Eopee. It's Rusty who realizes finally that Tiphia is simply in the wrong galaxy. This is a Baedeker of Outer Space.
