I have been able to find about this when browsing online is how to restore a whole photo library, which I do not want to do.
It is the only currently supported way. You could restore the iPhoto Library to an external drive, where you have enough free storage and then use iPhoto to search for the photos you want. I am keeping a drive with all old iPhoto, Aperture, Photos Libraries to be able to restore older photos.It is much more convenient than to have to search in a Time Machine backup of a library.
If you browse your Time Machine backup in the Finder, you will see an item "Backups.backupdb"
For example,I am seeing
Click through, until you see your Pictures folder with the old iPhoto Library. If you can find it, ctrl-click it, the use the command "Show Package Contents".
In the library package look for folders "Originals" or "Masters". Inside will be subfolders named for the year, month, day of the import. If you have an idea, when you imported the missing photos, try to track them down in the nested yoeatl and monthly folders.