I am not sure where you get a new, complete, flywheel that is heavier. Several people sell weights. I bought a zip-tye brand from someone on ebay. Mine is 10oz heavier. The weight also fits the 1998 YZ 400.
It was far and away the best mod I ever did to the bike. I believe retail is about 100.00 on the weight kit. It is well worth it. The bike feels quite a bit faster with it on. I know this sounds opposite of the desired effect, but much of the wheel spin is eliminated.
There was not much of a change at lower RPM's. The bike still revs quickly. People with several different riding levels have ridden my bike. This included "Studboy" who is quite fast. No one has ever said..."Take it off"
a flywheel weight is one of the best mods you can do, it makes the bike hook up i even run it for motocross.I have a stealthy 9oz on my yz and it got so much faster I started blowing turns (got going so much quicker) will never ride a mx bike without one again!