Most standard insurance policies have a clause that reads something like this:

"Participating in any sport or sporting activity for wage, compensation, or profit, including officiating or coaching; or racing any type of vehicle in an organized event."

When my son raced motocross, we signed all kinds of waivers and essentially said that we would be responsible for all medical costs associated with any accidents and indemnify the track owners and officials where we happened to be racing. We had great sponsorships for the bike and all the related equipment, but all participants realized they were participating in a very dangerous activity and in the event of an accident apologies were given and all parties were to walk away. We were fortunate that he suffered nothing more serious that a few sprains and slight concussions.

That said, K&K Insurance does offer insurance for all kinds of motor sports from dirt bikes to drag racing. Their web site lists all their coverage and also states the minimum account is $1500.

http://www.kandkinsurance.com/

Good luck with this one,
Glenn