Andrew R Stewart 

The erector set style of crimping tool is made of plate steel, hand filed/ground steel form and all thread rods with other bits that should be obvious. The plate is about 3/8" thick, I went to a local steel supplier and sorted through their scrap for it. The all thread and other bits were local hardware store stuff. The form is held onto the plate with machine screws.

I've seen dozens of production bikes with chain suck damage as you describe. None that I can remember have produced cracks including many Al frames. But 753 is somewhat more brittle then other materials so I don't know the future reliability of your 753 stays. My initial thoughts about "fixing" this are to patch the damaged spot with a thin and well curved plate, silver brazed on. But this will make ring clearance more problematic so some home work is advised WRT clearance. One could fill the dents with brass but this would only be a cosmetic solution, IMO. If this were my bike (and I have done the same chain suck damage to a tandem I have) I would just ride it and monitor periodically for more problems (and I would replace worn rings/chain as keeping them in good condition is a big way to avoid future chain suck just as always soft pedaling is the technique solution too). Andy