First you don’t want to use the original as a former – you need to make a solid wood mould to hammer on or, more skilfully roll and hammer off the mould until nearly there. Some shapes are better hammered from the inside.
The starting sheet needs to be only just larger than the final item, perhaps needing to be an odd shape. It will stretch anyway. The ‘extra’ you have is a problem to fold out of the way.
Gradually as you form it before the excess gets out of control, you can either use a ‘shrinking tool’ in a press or very skilled workers can do it with a special hammer with a rough surface.
If the extra gets significant it can be cut out and welded shut.

My late neighbour was a coach bodywork repairer and he could make a complex shape in minutes for the private jobs he did at weekends. Was a bit noisy living next to him though.

Hi Daniel, As above, a REAL panel beater would use special tools but for us meer mortals cutting out a section and then welding the 2 edges together is easier IF you can aluminium weld. Regular annealing may help ? For getting a development of the shape first start with a large sheet of paper or light card and fold it over the pattern tank cutting as needed. Good luck. Noel.

A shrinking hammer has a cross hatch pattern on the face. When the metal is struck some goes into the hatch.

You use you wrist to work the hammer not the whole arm

Have you got the right grade of Ally plate?

Aluminium motorcycle tanks are more likely to be made of three parts, left, right and underside. Someone who knows how to use an English wheel could make the parts. Unfortunately learning how to use one could take years of practise. There are tank makers who would make one for you. As for your piece of aluminium, it looks like a sculpture by one of the modern artists who charge millions.