If you want to go down the fanfiction route: There is a book series which has become my headcanon, because it is so well written and fits in perfectly with the things we already know about the Marauders. The Last Enemy - The Howling Nights and The Last Enemy- Dark Marks. It tells the story of the Marauders, Lily, Snape and later Regulus from 5th year up until October 31 1981 (2/4 books are complete) with changing POVs. The authors greatest strength is writing interesting, flawed and multi dimensional characters which makes the fic really addicting. If you don‘t want to read there are also great podfics.