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Yea unfortunately , they have all discontinued the weights and heavy flywheels for the older models , I guess they figure if someone was going to add one , they would have done it before all the new models have been added
With that said ..... About the only other way is to buy a Steel Clutch Basket , it will give some rotational mass , which is basically what the weight does , though the basket is not as drastic a change , it will "help" ...... other than that , you can also go the route of the Rekluse ... though I personally do not like them , they will help with stalling ....... depending on how tight of trails you are riding , you could also maybe gear down a tooth or so , for a example , my 450X does not have any flywheel weight , but I did put a HotCams Stage 1 (all bottom and mid power which is better for trails) and went from a 13/51 , to a 13/52 and it made a huge difference when it came to lugging the bike .... , on my 02 450R I do have a flywheel weight , and when trail riding it , I run a 13/52 , and that helps it a bunck , then I just swap gearing to 13/48 and track ride it .... , though they are both 450's , the principal is the same , so that is also a couple options to explore ....... is maybe a cam swap and/or gearing .....
As far as I know , the 250's did not have much variance thru the years (the X has a heavier flywheel , but I do not think it will fit the R , due to case clearance issues ) , but with R flywheels a different year won't really do much , if anything at all as far as I know , the R is not the best trail bike , the X is just better suited and that is why it was made to be a trail bike , but it is not a R , so don't plan on going to the track with it , and the R would already have a lighter flywheel from the start than the X would , so you are fighting a uphill battle from the start ......
Basically with a older model , it is going to be tough to find a weight or heavy flywheel , unless you stumble across one on EBay .....