Have you done any swiming or can you? I started doing this after one of my concussion because it has no impact but is still a good exercise. Just start slow if you don't want to get your heart rate up and if it goes well you can start swimming longer and faster. I still do mostly light swimming since I still can't run because of the impact or ride a bike because of balance issues. It might not work but why not give it a shot and see what happens.

-Soccer14