It may be worth thinking about the type of Aluminium you want to forge. As an ex-biker I know forged pistons are softer and expand more than cast pistons. There is less silicon (if any) in a forged piston whereas it is used in cast pistons to reduce expansion - 9-17% depending on application. If I were to reach for some scrap to heat I think I'd look for some rolled plate or extruded bar before I reach for an old casting. The silicon limits the plasticity/elasticity you need in a forging - I think you may be looking at cracking as a problem if there's too much Silicon. I don't know a way of reducing Silicon content in a smelt other than dilution.
There's nothing to stop you from casting some melted extrusions like 6082 to get the shape ingot you can control, and then hammering the heck out of it. I'd love to see what you decide to make though!

D