I have an old CME XKEY midi keyboard. Its black keys are getting brittle and broken at hinges. I designed new black keys new scissor like replacements and key holders. Idea is holding keys on their place with TPU printed Z brackets, using Metal U bars intact for stability and another set of top brackets for leveling. TPU is required for Z parts. Rest is up to you. You will glue Z parts to keybed, and glue the top of Z parts inside black keys.

You need 15x prints from each STL. Use polymere glue on keybed. And solvent based paper glue on keys.