Re: Fiberglass tank...
I have a bit of experience with fiberglass gas tanks on boats. If the tank is made from polyester resin, (aka common bondo or other brand maple syrup color resins), the alcohol in the gasoline will eventually cause the tank to fail.
New "fiberglass" gas tanks used for gas with up to 10% alcohol are made from epoxy resin. If the tank is epoxy resin (usually clear or slightly yellow) then it will hold up to the alcohol in gasoline.
There are practically no marine gas tanks made from fiberglass anymore. They are all rotomolded plastic, or welded aluminum plate. The only sure fire protection from leaching plastic resin into your carburetor, and destroying the tank with alcohol fuels.
My guess is it is a common polyester resin tank. Lining it with one of motorcycle products meant to seal rust inside of a rusty tank would probably make it safe for todays gasoline.