We ran a 6 oz on our 98 for a while. If you have a friend who works at, or a local machine shop....they're not hard to make, give them the flywheel, tell them the maximum diameter your cover/case will allow. We have a friend make ours, usually $40-50. We managed to fit ours with the addition of about 3 gaskets to shim the stock cover outward.
Yeah it helped a lot. I still wanted some snap so I didn't go too heavy but it made a nice difference. They have a good chart to recommend the right weight for your riding conditions if you're not sure what to get. I used the Aggressive Woods, Novice MX category and like it.