I don't really know the piece extremely well, but a great piano student devises his own exercises to apply to his repertoire...
That must be quite easy to say. Or type. We can begin to devise exercises, when we know the fundamental problems at work. The fundamental issues. Until then, we are working in the dark, groping at every possible thing. Well, people succeed at that. Usually after about thirty years of growth and maturity. But knowing a few fundamental things can help turn on a few lights, and we can benefit from the knowledge and experience of others before groping around, and possible breaking or harming things. Like ourselves.
The first fundamental thing to know, from which all else derives, is, how to touch the piano? Like Godowsky said, a "pulling" touch, or like stroking the cat, or perhaps like the sensation of erasing pencil marks. It is actually a subtle thing that can be learned through exaggeration. All the fingers touch the piano like that. Yes there are variations. But those are just that - variations. Variations on what? On the fundamental.
Once we do that, we are free to feel the most important muscles at work. Trilling with 4-5, we will feel that fleshy side of the hand, the far side from the thumb, working, and it will feel good. The hand will feel thick, or hot, like blood is flowing through it, like an intensity. The speed will come, with the ability to connect with the true use of the muscles, and let them go, or use them, whatever we see fit.
Walter Ramsey