This is pretty messed up.

“If you own a custom bike made before 2006, you don’t have to get rid of it. Also, it’s perfectly legal to build yourself a custom bike, as long as you do it from a pre-approved do-it-yourself kit. The real drawback is that you’re allowed to own only one custom bike in your lifetime.”

EPA: Control of Emissions from Highway Motorcycles; Final Rule (PDF)

ABATE’s page on the subject