You might be able to repair them by simply using the PCB from the PS/2 keyboard, and simply mounting it in the USB keyboard housing. just look for the same model keyboards as you have with USB, as the manufacturers redesigned them to use the same membrane internally, the same mouldings and thus the same PCB with a COB on it, and the interface to the keyboard will be the same for the same keyboard layout, and the mountings will be isentical, with the same pitch, same hold down strip and hole spacing.
Otherwise you need to find some silver conductive ink used for windscreen demister repair, or go to the art supply stores and look at the silver pens that write conductive traces. If you just get the ink then get some Rotring pens ( large tip types), some solvent for the ink , and hope the silicone bulb in the pens does not dissolve too fast in the solvent.