At the very least, polish up the bad areas of that ground strip and tin the entire length with a solid run of copper... use a piece of stripped stranded wire as your base to cross the bad part.
As far as the mess of orange wires on the back - they need to be redone as there's all sorts of 'gunk' left there - needs to be cleaned with alcohol and the replacement wiring should be done more cleanly, following the tracks as best you can and not running wires too close - certainly use the right size wires, not oversized ones.... otherwise, this is a poster version of how not to do it (to be honest)
Again, check those continuities, don't be surprised if you fine a bad one or two in spite of getting sound out of the synth...