If you are missing some fins, you should replace the flywheel anyway.
The fins are only a good enough way to restrain the flywheel if your anti-rotation tool grips at least six of them evenly. Briggs made a special tool for doing that with their engines. Realistically, that isn't something the average home repairer can make. Remember, the nut isn't all that hard to remove unless it is rusted on. If it is rusted on, that problem can be eliminated by Penetrene. (WD40 is a poor substitute.) If you can make a very rigid socket spanner adapter and smack the end of it circumferentially with a heavy hammer, you may get lucky. Really, though, the answer is a rattle gun if it is a difficult one. You also need a six-point impact socket, don't waste your time with a 12 point socket, and the socket has to fit the nut tightly.
