If you can run iPhoto at all on High Sierra it must be iPhoto 9.6.1. And this version has only been available from the AppStore. Are you signed into the App Store with the same Apple ID you have been using when you first installed iPhoto? The purchases are tied to the AppleID you used when you first purchased iPhoto or accepted a preinstalled iPhoto version to your AppleID. If you have a different AppleID that you used previously, try to sign into the AppStore with that AppleID.
I tried to copy my saved iPhoto folder (with years of photos)
I am not sure what you mean by that. If you just copied an iPhoto Library in the Finder there should not be a warning about missing fonts. Have you been trying to open an iPhoto project for a book or a slideshow?
If you are trying to open an old iPhoto Book project, you are out of luck. You can no longer order books from iPhoto.