125B arm wont bolt into a 250A 125B arm is way smaller .
Some of the differences off the top of my head betweem 250 A and 250 B are bore and stroke ,Cyl head , chamber, muffler, air box , swinging arm , shocks, forks , shock mount position , front number board , Cdi ,seat , brake pedal, front axle , rear axle , Early A models have different tank , kickstart and shaft , magnesium rear hub.