Hi Mr Wizard,
They sure don't make it easy to work on them.
It's great you noticed the crack and that you were proactive about fixing it. Lots cheaper than a new motor.
If your motor is steel--then a steel tank, but if it is aluminum--then best to stay with aluminum.
What happens is, over time, the plastic depalmerizes. That means the long chain molecules break up and become short. When they do, the plastic become friable. Heat hastens that process. I don't think plastic welding is going to be a good idea. Yes, it will fix the existing cracks--but there will be more in future.
