Ask HN: My father had a stroke, know interesting rehabilitation startups?

Sorry, not a direct answer. You might want to look into Lions Mane mushroom extract. There is a double blind study out of Japan that shows nootropic properties (regrows nerves). Good luck to your father.

Another not quite direct answer, but at least in my part of the world these kind of companies are more often "spin-outs" from university research departments and institutes rather than "start-ups" from the general economy.

It might be worth having a look the universities doing research in this area, then seeing if any of them have a group in a business incubator.