In the US there are plenty of service centers that specialize in replacing lithium batteries in cellphones, tablets, watches, and e-readers. Finding a replacement battery is the hard part. I do not see any listings on ebay today for a Kobo sage battery, but if a fellow member of this forum who has done this before could provide the mAH rating (1200?) of the battery and its dimensions, the rest of the process should be easy to accomplish.
A youtube reviewer mentions a power cover. Is this a way of supplementing the charging process?
https://www.youtube.com/watch?v=Ufd7CjhNkfI