Remove the oil from the tank.

I suggest pulling the tank, removing the stopper and using some acetone to clean the tank out. Soak the clunk in some acetone also to get that cleaned out.

At the same time CLOSELY inspect the tubes that go through the stopper for corrosion (replace if needed OR if they are suspect) and replace the fuel line(s) inside the tank at this time also.

If the tank is not wrapped in foam, I would do that also.