Try Ace Hardware or a full service hardware store for the key.Looking for the woodruff keys for the clutch shaft. I'm tired of so called "Big Boy" stores charging $6-$12 to ship a item that weighs less then the envelope
80 Piece Woodruff Key Assortment @ Harbor Freight Tools
Storehouse - item#67584
http://www.harborfreight.com/80-piece-woodruff-key-assortment-67584.html
20% off coupon eases the pain
Not sure if these would be the right size.
Nice site, looks like a father and son might have found common hobby.