Now the obvious thing to do would be to get a sheet of glass, some 400 grade wet and dry, and rub away to flatten the flange. Obvious, but not really the best way. A more effective solution is to use an accurately made tool to apply the opposite pressure to push it back into shape. Not only will this bring the flange back to true, it will restore the shape of the rest of the body which has been distorted.
I made the tool a few years ago and it has rescued quite a few carbs from the scrap bin. The base is a flat alloy block with a threaded rod fastened securely into the centre. The 'slide' is turned from solid alloy and is machined to fit the carb bore accurately.
You simply pop the 'slide' into the carb, feed the threaded bar through a hole which is drilled to be central to the inlet and then as you tighten the nut, it pushes the whole thing back into shape. A pair of of 5 or 10 thou feeler gauges (one placed below the mounting hole on each side), allow you to bend the flange past straight so that when the nut is loosened the natural spring in the metal will leave the joining surface and the body absolutely true.
With the aid of this simple device, it takes about two or three minutes to fix a carb properly. If you work on old bikes regularly, it's well worth knocking one up.
See this link for information on how to install an Amal carb: