There are usually loaner helmets available, but I would shoot the event host an e-mail to confirm. If shoe polish isn't being used, you can use blue masking tape on the windows (or print your number on a piece of paper and tape it to the windows). As for gas, I wouldn't run with a full tank. Gas is too heavy.
You might also want to bring an air tank to inflate your tires, since you'll need higher pressures. If you don't have one, you can probably borrow someone's or fill up your tires at the closest gas station.