Hi Degas, the best solution is to find an old Eko from the 80's (a lot of Eko from the 80's are built with ferro sonic pickups), then take off the pickups and put it in your lovely Ultrasonic...
I don't know exactly all the Eko that are built with ferro sonic. But, i know that the "Eko Kiwi" does.
like this one. http://www.2ememain.be/musique-instr...3%A9lectriques
Hope you already got your missing pickup.
anyway... i hope it help !