whizzers and many older bikes had fiberglass tanks. if done right and sealed with a product like Kreem, they should last until you crash on it.
as far as your cable end, if you mean the barrel end for the lever, or any end that is used as a "stop," an epoxy-type glue or JB weld won't hold up to the pressure.
here's how i made a simple mold: http://motorbicycling.com/f13/how-ma...old-21321.html
i'm also using the same method on the end of the clutch rod. i measured where i needed the barrel, then threaded the cable through the clutch rod, pushed the lever in, and soldered on a new end. obviously it's not adjustable, but it cleans up the frayed end, and gets rid of the ugly cable stop.