Its fuel evaporating in the float bowls so you need to turn off the fuel tap and run it dry.. My outboard started and ran on E10 this morning. The fuel in it and the extra fuel I put in it was bought last September. Dry carb, ran out by turning tap off 50m before stopping before storage, sealed fuel tank.
Well, it took nearly 50 years for condensation to eat my fuel tank. If you provided dimensions I am sure there are suppliers who can make a polypropylene tank to fit, but then we dont need the impact resistance a plastic tank provides over a steel tank.. basically theres a class of impacts where steel tanks split at the seam while plastic tanks bounce back. At a higher impact level both fail anyway.