Spent two evenings desoldering corroded parts on the Black Hole Sys80 cpu.
Used sandpaper to get rid of the corrosion (forgot to take a before picture).
Finally used vinegar (it's an acid you need to neutralize the base fluid that has leaked out of the battery), scrubbed everything well, rinsed the pcb, ..
Now it's ready to solder in new parts and check all traces. The thick trace at the bottom left corner is broken where it connects to the bottom left pin of the ic above it (of the 2 the most right IC).
Update: soldered all the new parts in place. Found two broken traces, fixed with jumper wires.
Now tested the cpu. It's dead :-( Will have to review all connections and parts to check they're soldered well and there are no shorts anywhere..