First it would be good to find out just where the problem is that caused the breakage. Then if you want to keep your old chaffer as a spare, here's what I did to salvage an old chaffer that broke and re-broke after I too braced it up. It's been a while since I did this so I'm going by memory on the tube size, but I think I used 1.5"x3"x.25" rectangle tube.
Then I cut a piece of heavy flat bar and drilled and tapped one hole for the cinch bolt, and cut out a section the same size in the tube to weld the flat bar in.
Then I had to notch the top so the chaffer would mate up to the rails.
I used a cutoff wheel and a plasma to cut the old broken weak tube out. (do this work in a well vented area dont breathe the dust or smoke) Then cleaned up the cut joints, tacked the new tube in and squared it up, then welded it solid.
Just an idea if you wish to repair your old chaffer.