umm a yz490 is old as sh*t, suspension isnt worth a damn, which if you did any real dirtbike riding you would understand. trust me, if that guy is going to trade for a warrior you had better jump on it. You wont need to get a bigger bike after this, youll never grow out of it. Now granted they do need thier valves adjusted more than a 450, considering a 250f pulls about 13,000 rpms. and the 450 pulls about 11,000, and you wont need to rev it that high simply becasue its a 450, and it will last longer. But if you just get a set of stainless steel valves you should be ok, now if you fireroad it that bike wont last long at all, well, no motocross bike would anyway so thats irrelivant i suppose. But just take care of it and change the oil every 5 rides or so and you will be golden, just i would get around to changing those valves soon, no matter what they guy you get it from says. THye are only supposed to be shimmed three times before they are replaced. And since the bike is so old you might consider getting the valve seats recut