Do you need a top end in that 500?
I ask because my nephew (130Lbs) on his 2003 CR250 will get ABSOLUTELY destroyed by me (231Lbs.) and my 500 in a drag race. What gearing does the 500 have?
One thing to keep in mind is that the '03 CR250 is reported to make almost 52HP thats REAL close to the power output of a 500 (60HP) - Don't feed me that stuff about 64HP. I dyno'd mine after a pipe and reed cage and it was 66HP but loads more powerful than stock. The 250 has HP but gives up torque do it's small-ish CID. There is also close to 20Lbs difference between my then stock (238Lbs.) KX500. Don't believe everything a manufacturer prints
The CR250 weighs in at 218Lbs.
SO... if we take weight, power, torque into consideration that 250 still shouldn't be running too close to a fresh 500, but it may run closer than you expect.