Well unless you still have 5.0.1 on your old MacBook Pro or you have access to another computer with 5.0.1 you are out of luck. There is no way to revert to an earlier version as the App store only has 5.0.4. When you opened the project the first time in 5.0.4 did you select open as untitled? If you did then your original file is intact and if you can access a machine with 5.0.1 then it might open ok if the corruption occurred when Motion updated the file for 5.0.4. Sorry the news is not better.
I had a problem with a 5.0.3 project that would cause 5.0.4 to crash. Apple asked for a copy of the project and later told me it was because there were some empty text layers. Apple is aware of the issue and is working on a fix.
In my case I still had 5.0.3 on another Mac and could clean up the project. If empty text layers might be the problem, do you know someone with an earlier version who could open it and remove the empty layers? I'd be willing to give it a try if you'd want to send me the project.