Motocross photography is one of the few automotive sports where an expensive long lens are not really required. As a spectator you are close to the action and in the case of the Hawkstone International there is plenty of it for you to capture. A medium telephoto lens up to 300mm will work fine in most circumstances, with the standard 70 - 200mm f2.8 being the best choice.
Whatever lens you have with you on the day you will require a filter attached. This is not to enhance your photography, but to protect the lens from the copious amount of flying dirt. A clear glass /UV filter will suffice.
If your camera has the ability to record sound files then use it. All the pro cameras have this feature. The lead riders might be obvious to you on the day but later when you are editing and they, like the other 50 riders that are covered in mud, will be hard to distinguish, so a sound file attached to the image helps a lot, “Shaun Simpson leading on lap two”.
I like to make sure I have clean pin sharp pictures to work with so will keep the shutter speed quite high at the beginning of a race. Once I’m happy that I have a good number of keepers from different angles I will then start to get arty by slowing my shutter speed down to get a blurred background and a feeling of speed into the shots. With head on jump shots you won’t see the background much or the wheels rotating, so push that shutter speed up to over 500th of a second to keep the picture nice and sharpe.
It’s best to keep your ISO on automatic. This will help keep your exposure constant throughout the day as the light drops, helping with your workflow in post production.