I still say it's about the flow. A good carb, good flowing exhaust, and the right cam will do wonders. A polished head with big valves would be great, but I can't say that it would be the best bang for the buck. It costs money, and it won't help unless you have the other goodies. A carb and exhaust are probably the biggest help. Obviously, the flywheel and rod are important for safety, but I don't think the billet flywheel offers a lot of performance. I have always wanted a motor that revs really fast, and I remember expecting my motor to sound like a sprint car revving when I built my first clone. Well, I was disappointed. Obviously, the safety is important, and I had piece of mind. It was just a slower revving piece of mind than I had hoped for.