JS,
Welding on a ductile iron crankshaft wont work. You would be better served to make a larger diameter sleeve of the appropriate length and bore the pulley to accept the larger shaft. You can easily turn one on even a mini lather, and counter bore it to slip over the extsiting crank and secure it with a bolt through the extension or a key and set screw in the side of the extension. Unfortunately I dont have the capability to make a simple drawing, but it should be very easy to do and will be better ballanced and extremely durable. This repair should easilly last the life of the machine and can be moved to another engine if nessessary with minimal effort. Just sayin...
Bob