FWIW, the first generation aluminum frame was '97 - '99. Body parts and other things iike brakes and wheels will almost all interchange on those years. There are differences in carb, ignition, pipe, and cylinder porting on the '99.
From '92 - '01 the CR250 engine was based on the same design, but it evolved a little every year. Some parts will be interchangeable trhroughout this range, but be careful when mixing and matching cylinders, pistons, heads, ignitions, etc... '97 and '98 motors might be identical. If not, they are extremely close.