There are some fanatical Japanese tuning/customization shops, both for motorcycles and cars. One such tuner is known as Sanctuary. Sanctuary does complete rebuilds of older Japanese superbikes, including engine rebuilds, cosmetic restorations and modern performance parts. This Kawasaki Z1 is just one example, and includes more parts/upgrades than we care to mention.
Many of these bikes start with a reinforced or customized frame, and build from there. Wheels, forks, brakes and carburetors can all be transplants or upgrades as well. Follow this link to Sanctuary’s website if you would like to browse around yourself.