Sony CMD-Z1 (and plus) batteries QN-022BP and QN-023BP measure cells replacement

[sv3ora]

Hi I have 2 sony phones

CMD-Z1 (battery Li-ion QN-022BP 7.2v 730mAH)
CMD-Z1 Plus (battery Li-ion QN-023BP 7.2v 600mAH)

Both of these are 2-terminal battery packs.

1. I have a few QN-023BP spares but after 25 years the batteries are almost dead.
How can I measure which of them is more alive than the others? Load them with some kind of resistor and measure the voltage? Please give me some info on this.

These packs can be opened easily, so I think I can expose the cells inside and possibly change them.

2. Suppose I change the cells with Li-ion ones, can I use the same sony charger used in the original batteries or there is some kind of "magic" that must be used on proprietary companies cells?

3. What if the cells I get are of higher capacity, can I still use the charger I have and wait for more time for them to charge, or I should not do it at all?

4. What If I find a Li-Po Cell, can I use the Li-ion charged to charge Li-po or a special one is required?