I think that the question I would be asking myself is this.
Appreciated, that the keyboard may have sentimental value to you, but bearing in mind it's age (circa 2009) and what you have described, I'd be considering the overall cost of effecting a successful repair, if you aren't able to carry out such work yourself
Unfortunately, the nature of old, brittle, badly connected circuit boards and availability of other components may make such a project unfeasible. As fast as you fix one issue, you can be pretty sure that others will follow.
My own view is that Money Pit projects are a bad bet.
For what it will likely cost, go buy a working keyboard. Far less hassle.
Just being realistic