Has to be some company out there that specializes in insuring vehicles over $150k.
But on a serious note you'll often see people just taking the risk of running their cars at the track, some without a plate. Just an easy way to go about it
What would be interesting is polling people who do go to the track and see how many admit to not having a special insurance for track purposes. I bet most that are truthful won't have anything aside from regular vehicle insurance.