composer install without 'composer.json', is that possible?

Thats like running php without php installed.

Yes, I think you can composer install with only the lock file, but you will not be able to upgrade or change any packages

I think I would put the repo in a new folder, remove composer.lock, copy composer.json from a vanilla project that is the same Laravel release, run composer install and then run the project and see what issues it throws up.

You'll quickly start hitting missing classes which you can then composer require to pull them into your project

Once you know what classes are missing, you can refer to the previous lock file to determine the version you specified

As an alternative, just build a new composer.json file as needed. Get the information from lock file.

@SNAPEY - Thank you. I've managed this problem by doing this.