I remember my father had one of the portable Ultra2's - it used two of those huge 6vDC lantern batteries that pressed up on a printed circuit board inside the "waterproof" case and the transducer was an old puck style Lowrance 'ducer on a metal suction cup mount.
You should be able to use a Lowrance 'ducer for it, the connection in the back is the same if memory serves. For power Dad's old unit was just a simple black/red line that plugged in to the rear (can't recall if it was a barrel style or an XLR style connection), either way all it needs is 12vDC, I think I converted his over to use UPS style batteries instead of the old lantern batteries.
I think it's buried somewhere in a basement corner of my mom's place, next time I visit I'll see if I can get you some more info.
Any way you look at it, thought - it's going to cost you money if it doesn't have the transducer. I'd tell you to save the money you'd put toward making it work and put it into a modern, inexpensive unit. They'll both have about the same features and the newer unit will likely have a better screen resolution - not to mention a warranty.