Depending on the type of dent, it may be possible to take it out. Is it a nice even round dent, or is the metal creased? It is sometimes possible to braze the end of a slide hammer into the dent & pull it out. Or you could try using a round piece of metai welded to the end of a rod suitably bent to press against the inside of the dent through the filler hole.
VERY IMPORTANT: - If you are brazing the end of a slide hammer onto the outside of the dent you will need to remove the tank from the bike, completely empty the fuel & then throughly expel all the fumes from inside the tank. If you don't do this properly you will have an explosion