Once you've placed a Motion project in FCP, if you want to modify it, I highly recommend ALWAYS right-clicking (control-clicking) on the Motion clip in the FCP Timeline (or Browser) and choosing "Open in Editor".
This will automatically launch Motion (if it's not running) and open the exact project you want to edit.
Upon returning to FCP you will find the Motion clip has been automatically updated.
If you modify and save the Motion project without following my advice, when you return to FCP it will show as offline and you will have to reconnect it. So don't do that!
One of hundreds of valuable tips on the very-close-to-finished Motion PowerStart from DVcreators.net, soon available in the lafcpug store!