How to to fix an overflowing water tank?

Hi

my water tank overflow pipe is dripping. I have managed to get the ball valve out (it was rusted and lossely held) through a hole but was wondering is there any way of opening or removing the water tank so i can get easy access to it inorder to replace the ball. Its a small old black water tank which seems to be sealed and access to the opening at the top is awkard.

Regards
rahim