SOURCE: MOTOCROSS ACTION
Under AMA rules, you must race the same motorcycle that you used in qualifying in both motos. What happens if you blow the engine? You can put a new engine in the chassis you raced, but you cannot race a different motorcycle. What happens if the frame breaks in half in the first moto? You can use a new frame, but have to swap everything over from the broken bike. You also cannot bring out a different bike to ride the parade lap, which riders want to do to keep the tires on their race bikes clean.
Factory mechanics have made an engine swap and even the frame swap between motos in the past, but in the age of accelerated TV schedules, it would be very tough to do today.
Article / post by: Jody Weisel