How much work have you already invested into your new iPhoto Library?
To access your old photos in the Time Machine backup you have to restore the old library to a folder somewhere on your disk. The procedure is described here: https://support.apple.com/en-us/HT202321
To restore photos:
- Quit iPhoto.
- Connect your Time Machine drive.
- In the Finder, open the folder containing your iPhoto Library. Unless you've moved your iPhoto Library, it can be found in Home/Pictures/iPhoto Library.
- Enter Time Machine. (Time Machine can be found in the Dock or in the /Applications folder.)
- Choose your desired backup. The timeline on the right side of the screen displays a purple tick mark for each of the backups stored on your Time Machine drive.
- Select the iPhoto library you would like to restore, and click the Restore button.
iPhoto may prompt you to repair your library if there are some inconsistencies in your files. If so, please allow it to proceed by clicking the Repair button. If you notice blank thumbnails, reopen iPhoto while holding down the Command and Option keys. When the Rebuild iPhoto Library window appears, enable the "Rebuild all of the photos’ thumbnails" checkbox. This may take a long time, depending on the size of the library
Then you have to make up your mind, if you want to keep the old library or your new one.
Export the photos from the library you do not want to keep and import them into the other library.
Or buy iPhoto Library Manager, it can merge libraries: Fat Cat Software – iPhoto Library Manager