Actually, you can... If you find a good set of ZXR250C Barrels and Pistons, you can transplant them onto a ZXR250A engine. No other changes required. A few racers do this as it gives them a slightly larger capacity (about 260cc I think). It's basically the same as boring the ZXR250A engine out as you keep the ZXR250A stroke, but increase the bore from 48 to 49mm. I've been meaning to do this for a while, but haven't got around to it.