While visiting my favorite metal supply house:
Escondido Metal Supply
I saw 4 big chunks of steel which instantly yelled "Flywheels" at me...
20" diameter x 16" long steel @ 33cents/lb each weighs over 1000lbs...
You need a somewhat large lathe........
BTW.. this place kicks ass... it stocks everything... sheetmetal, steel , stainless, aluminum.. box section, round and square bar, structural steel (making trailers?) etc....