The Dodge Tomahawk is powered by a massive 8.3-liter V10 engine, but it features a V-shaped configuration, not a traditional V-twin. The bike has an estimated top speed of around 350 mph (560 km/h). It's important to note that the Dodge Tomahawk was a concept vehicle and not put into mass production.
It's possible that newer models have emerged since my last update that might claim the title of the fastest V-twin motorcycle. I would recommend consulting up-to-date sources and motorcycle publications to find the most current information on this topic.