Is it a crack or a casting imperfection? I have seen casting 'lines' on flywheels that look like cracks, but they're not. If the flywheel was removed incorrectly it could indeed be a crack. If it was 'pulled wrong' with enough force to crack it, then the flywheel is likely also bent. If it's bent it will wobble and you can see it by turning the motor over. A crack can usually be seen on both sides, likely it would go through the material.
Replace a cracked flywheel. A casting mark is no reason to go to the expense.