Doing a little more research on the Kobo N647 battery. It used the same battery as the PocketBook 601 (which looks almost the same as a Kobo WiFi). A Russian site shows a substitute battery, a Nokia BL-6U (or one of its clones), with the leads from the old Kobo battery soldered on. (Red lead to positive, black to negative and ignore the third "empty" battery contact.)
The attached image was found here:
https://e-ink-reader.ru/batt_en.php (they show substitutes for several old eReaders here).
This battery (or one of its clones) can be easily found on eBay or Amazon.
https://www.ebay.com/itm/BL-6U-Batte...frcectupt=true
(Note: It looks like most of the eBay ones ship from China. But the one linked above is supposed to be in the U.S.)