I still need to try this, haven't done this in years. In theory, after you have created your final game. Go to where your game is stored in "Unity Games". Find your game and make a copy. Open the copy, it will look something like this:
Open the "Assets" file and find all the free assets from the Asset store and delete them.
Remember to create a text file with links to the free assets and add it to this file, zip it up and upload it for the game jam.
Hope this will work.
