The swing arm modification is probably the most challenging part of the metal work. It requires a hole to be cut out of it to make room for the motor. Many people have correctly commented that cutting into the swingarm can seriously weaken it, potentially causing problems. This is partially true, but the final design takes that into account - The placement of the AC motor helps to strengthen and reinforce the swingarm stiffness.
This step requires precision - the motor must sit perfectly in-line with the rear wheel, so that the chain travels straight between them without any flex or twisting caused by misalignment. We used both a square and straight-edge to mark the cut lines with precision.