Are there any other batteries compatible besides the original?

I have an iPhone 2G that I decided to charge and turn on for the first time since I got it like 5 or 6 years ago. It worked fine then. Now it won't charge no matter how long I leave it plugged in. I've tried multiple cables in multiple outlets but it only shows the Apple logo for a few seconds before powering down when plugged in and it doesn't even show that if it's not plugged in.

Anyway, I suspect the battery's bad since it's been a long time since I charged it and it's rapidly approaching 20 years old anyway. But I can't seem to find a replacement anywhere. Every listing I come across is out of stock. I tried searching with the part no (616-0290) too.

I'm almost positive the cells themselves can't be replaced in this one, like you can in a laptop, so that just leaves substituting another battery. The original is a 3.7V lithium polymer with a positive and ground wire. There is no wire harness or ZIF connector like on most other batteries. I do not know if there is any microcontroller inside the battery or if all the charging tech is all on the main board.

I'm thinking some other battery made for another phone or media player may be compatible but I don't know what specs to look out for besides voltage and physical size. Or maybe that's really all there is to it?

Does anyone know what would make another battery compatible with an original iPhone?