John in LA
You can replace the whole arm or you can buy just the end of the arm and cut the old end off and weld a new end on.
The ends are the ball and the socket and a stub of 2 3/4".
The weld on ends are 14.99 at TSC SKU # 26779199. I am not saying to buy them there just the first place I looked.
I don't think it would be advisable to try to reattach the arm over the old ball.
Your decision should be made after looking at the wear on the other end of the arm. I look at them as throw away wear items, but I don't weld either.