You may choose to search volunteer opportunities by events. You can typically tell how big an event is by it’s title. For example a country Grand Prix like Singapore GP or British GP is typically organized by the FIA as part of the Formula 1 World Championship. There are however two country Grand Prix that are not part of the F1 circus and they historically include Macau and New Zealand where lesser series are the feature attraction. City Grand Prix are generally used by IndyCar, events include Long Beach GP or St. Pete GP. However they also name events after the series like the Toronto Indy. MotoGP however uses a combination of city or venue Grand Prix names, like the Red Bull Indianapolis Grand Prix at Indianapolis Motor Speedway of the Red Bull Grand Prix of the Americas at the Circuit of the Americas. NASCAR and V8 Supercars are famous for displaying the event distance in their name like the Coca Cola 500 or Austin 400. Often the distance is covered over a period of two separate races.
My favorite form of motorsports lately has been Endurance racing. So I will list all of the races I’ve participated in like the Daytona 24 hours or Le Mans 24 hours, Sebring 12 hour or the Bathurst 12 hour, and the 6 hours of Bahrain, etc.
If there is an event of interest to you, please do not hesitate to get in touch and I will be happy to help you research how to volunteer there. Have fun!