The connector for this guitar is actually a 5-pin DIN connector. You can order an adapter from places like eBay or Reverb. You can also go about replacing the connector with a standard 1/4 inch jack connection, though I'm unsure if the on-board effects would work since they may require phantom power from the 5-pin DIN connection, I'm again, unsure of this.
It's clearly broken here, probably one of the capacitors or resistors is completely out of its value which introduces this completely broken low-pass sound
Look for a channel called "SPACE KOOK" playing this same guitar but not broken, and you will hear how the circuit sounds when it is functioning. Electronic components from the 1960s from any country may require replacing, especially capacitors.