I see how to trace a WORKING board, but not how to fix a broken one w/o a schematic then....
Yeah but now you have a schematic.
Let's say you want to verify the DRAM data pins on either side of the corroded tracks on your board:
On the DRAM connector you can see sixteen data pins, VD0-7 and VD8-15.
If you follow the corroded traces upwards, you see the buffers U4 and U6. These are convenient to check because the pins are in order; one side of U6 from pin 18 down to 11 is VD0 through VD7, and one side of U4 from pin 18 down to 11 is VD8 through VD15.
So you can touch these 16 pins, touch the other ones on the DRAM board connector, and see if they beep. if they don't, that trace in particular is bad.
Some of the corroded traces may not be used for data, but from memory the ones that run near the battery are. So there will be other things to check too.