Epoxy might not stick to the plastic tank, but there is some stuff called Seal All (Any ACE hardware store should have it) that is amazing. It will cure in liquid gasoline!
I have temporarily sealed leaks in the BOTTOM of leaking auto gas tanks with it! Just put some of it on the threads of the screw you put into the hole. Tap the hole and use a fine thread SS machine screw with a fender washer on it to provide lots of bonding surface. I'm sure this isn't a USCG approved fix but might make a good temporary repair, and probably isn't much different than the way they mount sending units in top of these plastic tanks. Good luck!