AccuBattery and AnTuTu. Make sure to run Android 10 so you have access to Mah readings. If you're using AnTuTu, charge at least 80%, otherwise the reported values will be ridiculously inaccurate. AccuBattery delivers somewhat reliable values for charges 30% or greater.
As for battery replacement I have no idea. Maybe you can find an original battery that has been cared for over the years. I would not trust 3rd party batteries, but at this stage of its life cycle maybe it's worth a try.