2 Other Cheap Alternatives.-
There's a flexible putty compound, comes in a 2 color strip, the one that needs to be twisted to mix well while strip color changes. When fully mixed wrap a very thin ring onto tank's neck, shape & compact with fingers till putty adheres well.
As plastic is smooth, will be a good idea to sand, abrade tank?s neck adequately previously for putty to hold well onto neck.
With finger dampen oil to the internal cap's threads contour, gently bolt cap back till seats nicely and forms a new neck thread, don't over tight cap or will pull putty off neck. Remove cap from tank, let dry putty till next day. Putty must be flexible during the thread formation procedure, if hardened no way...
Get a cork that fits inside tank's neck, drill a small middle hole, insert a thin 3/4" or 1" cannula/metal or plastic tube into cork to let air in and don't spill fuel out when OB is in operation, will be able to use the tank but not seal air vent, could get a second cork to close tank when out of operation. Will work ok if OB is not removed from transom and layed down...
Or buy a new internal fuel tank...
Happy Boating