Jim,
I think it is all to do with what plans you make it from. These engines are notoriously difficult to get running, and a major change like metal the bore and piston are made from could prevent the engine from ever running.
I used cast iron for the one that I built, purely because Jan Ridders, the designer, stated that the carbon content of the cast iron was the only lubrication that was needed on the engine. I do know that other designs do use lubrication, and in those circumstances, then I think ali could be used. But don't quote me on it.
I would tend to stick with what the designer recommends, then you know you will at least have a chance at getting it running, because he has already done it.
John