The real time clock chip takes 5V, so you can use either of the following replacement batteries:
https://media.digikey.com/pdf/Data%20Sheets/TADIRAN%20PDFs/TL-5101.pdf
https://media.digikey.com/pdf/Data%20Sheets/TADIRAN%20PDFs/TL-5902.pdf
The TL-5902 has more capacity, and has a lower discharge rate.
If you have a ROM 01 motherboard, you'll want to get the TL-5902/T which has solder tabs already attached.
If you have a ROM 3 motherboard, you'll wan tto get the TL-5902/S which is the normal button top.
Remember to check back in another twenty years!
tl;dr get the TL-5902
