Aluminium will be less useful as a roll cage than the original plastic one. It's not very flexible.

You basically have two options for roll cages, either allow a lot of spring and flex so that it retains shape, or allow none. The best materials respectively to provide these properties are plastic and steel. Aluminium will be lighter, of course, but that's irrelevant since it won't fulfil it's primary function. Try Titanium, if you can afford it, that would be cool, but still not as strong as steel.