I bought a "new" go pro off of craigslist and to my surprise, it wouldn't turn on, no matter how long I left it on charge. So I decided to look into buying a replacement battery. I had to wait a few months for ifixit to restock, but it was well worth the wait.
It was fairly straightforward. The only issue I had was taking the wires out carefully without damaging the surrounding parts; the part that had the wires in place was very tightly secured, so some force was required.
This solved my problem immediately! My go pro actually turns on now; this battery was a life saver. Though this model has the reputation to have a short battery life, I would suggest to invest in a battery extender/pack, or keep the charger on you at all times like I do.