The Afghan American novelist Khaled Hosseini’s best-selling début, published in 2003, is a sprawling yarn spanning decades and continents, with a narrative engine fuelled by betrayal and guilt. Turning “The Kite Runner” into a play must have been a challenging endeavor, but this new Broadway production does well, for the most part, by the source material, even if it can be frustratingly earthbound. Matthew Spangler’s adaptation, directed by Giles Croft, tracks the physical and emotional journey of Amir (Amir Arison, in a marathon role), an Afghan refugee in the United States who is haunted by the fact that he deserted his best friend, Hassan (Eric Sirakian), at a time of great need. The first act is sustained by efficient storytelling; the downgrading of the character Assef (Amir Malaklou), however, from the sociopath he was in the book to a garden-variety bully, is indicative of a general timidity on the production’s part.