i did some aluminum casting when i was in highschool. we used all sorts of stuff for the aluminum, but mostly it was old aluminum electrical wire [high tension 0 & 00 gauge stuff]. pistons are a great idea, as long as you get the cheap plain cast ones [not the high silicon content hypereutectic ones. . ]
we used petrabond sand for the forms, but after it was used a few times, it wouldn't hold a shape properly, so we also did some "lost wax" casting, but used blue styrofoam unstead of wax