The only practical way to replace the ball joint on a stock arm is to cut the existing ball joint out of the arm and weld in a threaded bung. Use a threaded ball joint from somewhere like lonestar, or If I remember correctly a tie-rod end off of an early 70's VW bug will fit as well. I am thinking it was the RH inner tie-rod end. You'd have to check the ends to make sure though.
You might also be able to send your arms to ATV Racing in Phoenix for them to repair. 623-516-8640
Chris