You might try this link to the app in the Mac App Store:
https://apps.apple.com/us/app/motion/id434290957?mt=12
If you have previously purchased the app (obviously), you might have to log in to the App Store and find the "Purchased" button to access your previous purchases.
Next time you upgrade your macOS or the Motion software, heed the instructions in this support document -
Motion release notes - Apple Support:
https://support.apple.com/en-us/HT202203
In particular, the part at the bottom that describes how to Backup the Motion application.
All of this would probably be unnecessary if you have Time Machine backing up your Mac.
Back up your Mac with Time Machine - Apple Support:
https://support.apple.com/en-us/HT201250