Go to the AHRMA web site (ahrma.org) and become a member, visit the online classifieds and so on. I've been racing vintage for the past 5 years, and also function (barely) as an AHRMA midwest coordinator. The two bikes you mention are widely available, especially the Honda, which is also the better bike in my opinion. You can find everything you need, parts and cosmetics, without much effort. I'd be glad to answer any of your questions if you want to email me directly at markthomps@juno.com.
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'81 490 Maico (King Kong!), 2000 CR250, 74-1/2 GP400 Maico, buncha street stuff