I don't have insurance for a race car but the Buggy in my Avitar there is insured with Motorsports Insurance - Philadelphia Insurance Companies
I think I called Grundy Classic car and this is who insured it. No body else would. They will write any policy value you want and just set a bill cost to it.
If you don't get any other specific replies call them.
I don't know that I've ever heard of insurance for race vehicles. You can get it for "recreational vehicles", but the minute you are competing in an "organized" event, the policy becomes null and void. I know there are companies that will cover race cars during transit to and from an event, but again, once it's on the track, no coverage.
In the case of BAJA Off Road vehicles - I think most of those questions can be answer by this guys INSTANT INS.
Instant Insurance has been the Off Road insurance company in Baja for many years, I'm sure the might have a solid answer.