I would start by tracing the outline of the lid on a large piece of cardboard, then rotating the lid so the flat spot is sitting on the trace of the undamaged part. That will show you the location and extent of the damage. Cut a piece of wood to the correct inside diameter of the lid, and another slightly larger than the correct outside diameter - you will have to pull the dent slightly beyond the final diameter because, due to the elasticity of the metal, it will spring back when you release the tension. Position the two wood pieces on the inside and outside of the dent, and use a clamp to squeeze them together. Use the traced outline as a template to judge your progress. You may still have to do some hammering to fine-tune the result.