Model Aviation Products make very nice rings that are perfect for the job. They fit through the hole and have a flange with lots of little holes drilled to hold the glue and set it securely. I am having some tanks made at the moment and am using the MAP rings on them. The FEJ tanks are not naff, they just need finishing off. I would never use a fibreglass tank without fitting an aluminium ring to strengthen it. As yours have cracks you will have to pray that the glue fills it and makes a waterproof joint. You may be able to seal any tiny cracks with Cyano after the ring is fitted. You can use Hysol or I have found Acrylic glue for this application to be better as it runs into cracks.

John