I'm looking at switching to an aluminum flywheel as that would save 23 lb over a cast one. Cast flywheels typically weigh 38 lb, steel ones weigh about 30 lb, and Al weighs about 15 lb. I hear it's a little hard to started from a stop with Al, but overall it should really help performance. People spend a lot of money to save a few grams here and there from the rotating assembly, but with the Al flywheel, you can reduce rotating weight A LOT.
I would get a cast replacement or if you can a billet steel. If you're making a race car aluminium is good but if you're making a street/road car the extra mass of the steel/cast flywheel is beneficial, a lot of kinetic energy in a spinning flywheel.
