Both Might Guy and Rock Lee work for this idea since they are like literally the same character but I think Might Guy would be better since he was older.
After releasing the 8th Gate, Guy failed in killing Madara. Naruto didn't manage to get to Guy on time before the truth seeking ball destroyed him.
When Guy's consciousness returned, he found himself in a body of a baby in a new world.
I would make Guy an orphan since I think it would be easier to write it.
You can make Guy quirkless or give him a quirk that gives him an enhanced body that will allow him to become stronger than a civilian with hardwork.
You obviously want to keep his personality the same.