At just about any MX track in New England (including Southwick), you can show up at race day without any membership, pay a day fee, pay for your class, and go racing. There are quite a few organizations, NESC, NEMA, NEMX, Maine Motocross Series, etc.), but none of them require AMA membership. If you just go with a day pass, you don't accumulate championship points though.... I'm not sure about Enduro's though, I do think you need an AMA membership to join NETRA, but I could be wrong.