Not sure what your skill level is, how fussy you are, the overall codition of the bike, or budget but...
I would not try to bang it out from the inside! If you start trying to do that the next thing you know you’ll be painting the whole tank. Its not worth the risk. Then when its all done the paint wont match either.
At most if you can contain the bodywork to behind the tank cover- then i would take it off and fill it from the outside and paint it. Don't do anything that would cause you to have to paint the whole tank.
Talk is Cheap