Hi
That’s quite the find you have there:-)
Here’s what I would do
I would order an new battery for it as you are already halfway there to having it fully working!
But before you do that I would carefully dismantle it and take the logic board to a repair shop and have it ultrasonically cleaned:-)
Then tackle the rest with some isopropyl alcohol to restore it to its former glory!!!!
But make sure everything is working before order anything so while you are getting it cleaned maybe ask them to test it out.
I hope all goes well for you and your Mac
If you need anything just ask
Thanks:-)