I have had this problem also. PO left
outboard in a vehicle and closed the vent all under the hot sun. Tank burst along seem.
The outboard was a spare
motor. I was on Grand Turk and needed it without having access to
parts or supplies.
I just JB welded it. Clean it really well,
fuel drained of course. JB weld is still working after 3 months. I figured that it was best to take the
cheap route (and only possible one) and replace the tank later if I had a problem.
Jeff