To prevent Steam from overwriting your new 100% save with your old cloud data:
Often the most reliable source, as speedrunners use specific saves to practice certain bosses or routes. Look under the "Resources" section of the Binding of Isaac: Repentance page. the binding of isaac repentance 100 save file download full
A staple for game modifications. Search for "Repentance Complete Save" to find files that are community-vetted. How to Install the Save File (Step-by-Step) To prevent Steam from overwriting your new 100%