Scooter chassis have the structural efficiency of ornamental garden gates generally. If you wish to alter a mild steel bent tube type thing then use mild steel, the use of fancy alloys is at best a waste of time at worst potentially dangerous due to possible welding problems.
Why not throw the horrid structure away and make a new one from wood. I have not lost my marbles, Red Shift was a motorcycle made from plywood sheets formed into a box section type structure. It has the same stiffness to weight ratio as any fancy alloy, is cheap, easy to form, fabricate with cheap tools, non conducting and you can claim ecologically friendly.
Yours OMG