im not 100% sure about the cranks, but i know the crank bearings are the same. this makes me think the cranks are the same, and if so i dont know why the rest wouldnt be about the same. only problem i see is that the 250 is gonna be smaller comin out of the cases. the cylinders wont interchange. i bet the gears and all are pretty close to the same, but the cylinder isnt gonna bolt right on there. as for making the cr500 engine fit into the 250 frame you shouldnt have much problem unless its an aluminum frame and then you have to have someone who knows aluminum make any needed changes to the frame. i bet case wise they are almost the same.