✦ You'll need to un-crimp that part in the center and lift the "eared" part off. You can probably best reattach with commercial grades of epoxy, or (after filling the upside-down cap with water) low temp silver solder.

✦ An option might be to find a large cross-section o-ring (~3mm) with the same ID as the diameter of the "eared part". That would more easily stretch over the "eared part", and come to rest in the corner formed by the junction of the cap and "eared part". This might contact the fuel tank opening on the rounded corner of the opening and thereby stop liquid fuel. 

Hope this helps.

Thanks, Richard.  Great advice.