Ren'Py game files are platform agnostic. So download the PC version, unzip, copy the folder called game, then delve into the older Mac app (right click, show package contents, navigate to /Contents/Resources/autoplay), replace the game folder with the one you copied from the PC version.
Caveat: check if the Ren'Py version is the same: Script-version.txt in said game folder. If not, you're better off by building a Mac port yourself with the Ren'Py SDK.
Precaution: always make a duplicate of the old app before you start modifying it.