You should be able to MIG weld it, using argon, if you can clean it well enough. Second-hand parts may not help all that much, since the main problem seems to have been electrolytic corrosion between the aluminium flat part and the steel Allen screw. The corrosion deposits built up in the gap between the flat and the screw, which eventually caused a tensile failure of the aluminium flat in two places. Unless it has been kept dry, a second-hand one is likely to have much the same problem. Unfortunately the hole is now much larger than it was originally, so if you repair it by welding, it will have to be bushed back to the original size before you can re-use it. Might be a good idea to keep it dry in future, and use a stainless steel cap screw to keep the electrolytic reaction to a minimum.
