so far PTC provide some rest API command where we upload/ publish a project or send metadata information e.g. with Curl command, but so far, I know, there is no option to download the project e.g. to zip file. Today I checked PTC database, but I could not find such option.
I think following option are possible in your case:
1.) try to recreate the project for the old corrupted file
e.g. you can create a new HoloLens project and then from the old project try to replace the e.g. Home.js and Home.json (…Documents\VuforiaStudio\Projects\<yourProject>\src\phone\components )file into the new project (respectively viewName.js and ViewName.json) also all data from the download directory should be copied to the Upload folder (…Documents\VuforiaStudio\Projects\<yourProject>\src\phone\resources\Uploaded)
2.)open a PTC Technical Support case asking the cloud team to copy your project from the Experience Server directory on the Cloud instance and send it back to you