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I have no idea, but I know a guy on reddit who's archived most fanfictions out there. He helped me recover lots, even deleted fanfictions. Send him pm and ask him, he's your best shot.
reddit.com/user/nerdguy1138
There is a sub for such queries
Thank you for your suggestions! I don't have a reddit account, but I'll see if someone who does can post on my behalf. I'll let you know if I find it that way!
Update: Problem solved! The fic is by mugglesftw and is called "Why Time Travel Should Be Illegal and Other Morals."
This Harry Potter fan fiction was posted on Fan Fiction Net and rated T or lower. The main character was a Christian from the American South (probably a self-insert) who, one morning, woke up in the 1980s. Realizing that he had somehow time-traveled, he got a job as an elementary school teacher (a job he was already credentialed for), only to find that his kindergarten class contained one Harry Potter and Dudley Dursley. He ends up adopting Harry and also later marries Harry's caseworker, a Squib whose name starts with L and who has traumatic memories, and has other kids with her. Some plot points of this fic include: our narrator teaching the class to sing "You're Welcome" from Moana (which he passed off as his own creation), the narrator's wife becoming a school counselor at Hogwarts, the narrator refusing to engage in premarital sex with his girlfriend (later wife), there being assistant teachers as well as full teachers at Hogwarts (this was because J.K. Rowling Cannot Do Math), the narrator objecting to being called a Yankee/Yank because he was from the South, and the narrator's family later living in Hogsmeade while their kids attend Hogwarts. There was a lot of future-pop culture jokes. The fic was very long and was written in first person.
Edited by birdsinthewindow