I attempted to give a fellow COG member a replacement carb bowl screw once from the spare set I have. I had to give him the entire carb bowl and screw because the screws/bowls I had were a different size than the screws/bowls he had. I don't know how isolated an incidence this was but just be aware that there may be more than one "correct" size.
The screws that hold the bowls on are all the same size, the DRAIN screws are different. i've seen 3 different types, one for a slot head screwdriver (very early models) one that uses a larger hex,(early 90's) I'm guessing around 4.5 - 5mm,, and the later ones that use the 3mm hex.
On BIG caution I want to throw out there is DON'T OVERTIGHTEN the drain screws? they're tapered and can literally "drive apart" the bowl and destroy it with a big crack! steve