I would try to find a real woodruff key before I used the washer route. DBD is right when he says to check with hardware stores and cycle shops and even automotive supply stores as to a woodruff key assortment. Take the pieces you found to match up. Also take the dimensions (width of the slot, depth of the slot, and you can grind one that is too long to fit, but one that is too short to fit won't work worth a crap).
Unless the float needle is bad, you should be able to clean the carb without a 'rebuild kit'.
Lay a clean rag on a bench, take the carb off the bike and take the carb apart on the bench. unscrew teh cap that holds the slide in the carb body and let teh cap and slide hang on the cable. Make sure you have a clean workspace. Have a notepad or digital cam at hand. Make notes/take shots of things to give yourself a reference.
Take the carb apart, noting where everything goes. There will be an idle adjust screw on the side of the carb body right next to the slide. Leave it alone, no air or fuel passage is affected by it. Near the downstream end of the carb body will be a small brass screw head showing, in a little recess. That is teh pilot air screw. It will have a slot for a flathead screwdriver. Turn this in(clockwise), counting how many turns it takes to reach 'bottom'(don't crank it down hard). Make a note of this(usually 1-1.5 ). It will have to be reset when re-assembling carb. Now, remove that screw all the way(counter-clockwise). It will have a taper on it. Check this taper and that it has not got a groove worn into it. There will be a spring under the screw, which will prolly stay in the recess. You need to get it out, same with the o-ring that is in there. All rubber parts should be off the carb body or they may get screwed up by the carb cleaner.
Shoot carb cleaner through all the carb passages, paying special attention to the jets(brass orifices) and to the holes in the upstream carb throat. Remove the needle jet if possible(it is the brass tube above the main jet and goes out through the top of the carb).
Dry it with compressed air if possible.
Re-assemble.
I didn't include the whole process because I got to thinking about how much typing I was doing and how many little details are critical and so I wrote what follows.
I strongly suggest you be realistic about your abilities and find someone who has experience doing this to help you, or remove the carb and have a shop clean it. It won't be all that expensive(not nearly as expensive as screwing the carb up).