The series is based on the story "From the Apennines to the Andes" included in the novel 'Cuore' by Edmondo De Amicis. The plot-line was much expanded in the series, and many characters were added.
It was also insanely popular in Israel where it was translated as "HaLev" or The Heart, which is the title of the novel the original story is from. The show was probably the first popular anime in Israel, and it's lead character Marco even appeared in a bank commercial in 2022.
Extremely popular in Brazil, where it is know as simply "Marco", from 1979 (when it was first aired on open TV) to 1981.