Just like any other business looking for sponsorship, you need to:
1) Define your demographics - What is "massive"? You will have to clearly define the level of exposure, and how the exposure fits with certain brands' image and their advertising goals.
2) Financials - Make sure your value proposition has a good potential for great ROI for the brands and figure out the fees accordingly. Pricing in sponsorship can be difficult to figure out, and you will have to justify your ROI with good data.
3) Demographics - Figure out which brands/companies are good fit to the demographics that racing would expose them to. Reach out to them.
For reaching out, you should use personal connections, cold calls and with tools like a Media kit.
Hope this helps.