A welding is a casting so a welded lift arm will never be as strong as the one that broke even if a perfect weld has been done
So clean the old weld off , reweld it, grind the weld back flush then weld a brace across the place where it broke.
Old blades make excellent patch plated as they are a particularly strong grade of steel.
Once repaired, pay careful attention to the balance of the blades and condition of the spindle bearings
Lift arms break because the spinning blades create excessive vibrations