This site would never have been as informative as it is if it were a pay site. You will weed out more good than bad I'm afraid. You will get a very low number of new members and as people get new quads and get out of the sport this site will be nothing.
Edit
Leave the site as is. Add some paid advertised banners. Hell, start a parts store..that's what I would do if I bought this site. But, don't start charging individuals.