AHRMA has expanded it's race classes to include post-vintage non-current racebikes. Many of the guys who owned and raced Manx's and 7r's and the like are dead or the bikes are museum pieces. And most of the people who do are using repros, either part or all. When it comes to 500 GP 2-strokes, they're all pretty much junked (used up) and too expensive to reproduce. Most other dedicated or converted racebikes available for public consumption were built to last more than one season and are still safe to rebuild if you can find parts, old magnesium wheels notwithstanding. Chronic issues are well-known and watched carefully. That doesn't mean a race is never stopped to clean up oil on the track after an engine blows.