Sounds as if it is already a major burst hazard, Joe. Welding it skilfully with MIG will probably make it better rather than worse, but no engine with a cracked flywheel should be run. Bursting flywheels kill people.
Since so many of them seem to have the problem, it sounds like a design fault. Making a new, better-designed flywheel is the best solution. There might be a way to save the old ones - shrinking on a steel band around the outside is the obvious way - but I'm not going to endorse that idea without a whole lot of detailed information about the flywheel and the problem.
