If you're looking for manga similar to Mazinger Z (1973), you might like these titles.
After Koji Kabuto and Sayaka Yumi leave to the USA, it's up to the rash orphan Tetsuya Tsurugi to pilot the next Mazinger robot: the Great Mazinger. Under the direction of Dr. Kenzo Kabuto and helped by his adoptive sister Jun Honou, Tetsuya battles the Miccene Empire...
Ryoma Nagare, Hayato Jin, and Prof. Saotome pilot three specially designed combat jets, which can be combined together in different forms to form three different kinds of giant robots, Getter-1 (used for aerial combat), Getter-2 (for land-based conflicts), and Getter-3 (for undersea battles). They were assembled by Prof. Saotome and became Earth's first line of defense against the Dinosaur Empire, which is a civilization of reptile-like humanoids, who evolved from the now-extinct dinosaurs that roamed the earth millions of years ago.