Try Arbor Insurance in Ann Arbor - I can't find their number due to misfiling in the Rolodex.
They handle our policy and one of their carriers specializes in small business and machine shops. I"d be disinclined to mention race cars unless they ask as this might be high risk/no way insurance. They will NOT insure anything associated with aircraft, especially product liability.
We do race car parts for engines and they don't seem to mind. But if you do the car, they might balk. Premiums are based on yearly sales and they regularly/yearly audit you. Good luck