Oh, right, I forgot to include one thing: you need to install Git LFS (https://git-lfs.com/) to get access to the asset files. If you install it and then re-clone the repo, it should work.
EDIT: If git lfs does not work, I added a release (https://github.com/jani-nykanen/one-last-adventure/releases/tag/v1.0.0), where you can download the source code as a zip folder, and it should include all the asset files "normally". Then you can just copy the asset folder from there.