Since I used to OWN one (recently sold) I can tell you the parts are not that hard to find, just got to know where to look..the bearing is common for FIAT 600
if the diff is that bad, email me directly and I will locate something for you. the Bearing number is on it, and if the gears arent toast can be fixed.
I do know of another Zelle not far in restorable condition, maybe you might be interested in the entire car, fairly good shape.
BTW mine went to the island of PR.. not likely it will ever return to the mainland.
steveg. scg13803@yahoo.com