If it is the rear derailleur itself then a replacement derailleur is an easy find. Most of the Kabuki bikes used SunTour through the late seventies my Diamond Formula came with one . If it is the frame where the hanger is , there are hanger attachments. They are commonly called “claw” hangers and attach via the dropout. A picture would be helpful to see what exactly you are dealing with. Kabuki used common mech’s that are still available used through eBay or a local bike co-op. If the exact replacement isn’t available then others will work. You can put pictures in you album and then someone here can post them .