That is almost certainly a dead PCB. The water will have corroded the tracks and probably delaminated them from the board, not to mention water damage to components. The proper repair is a replacement PCB. Cleaning it up will merely make it look pretty.
If you want to keep the mechanical parts as spares I'll give you a few groats for the solar panels - if they survived. I've got a post on the wanted forum for Tacktick bits.