There is a kit from RB mods that has an engine sealant in it. Other wise you cn go to any auto store and get the high temp RTV. Take the carb out of the motor, and apply a thin band of the RTV at the base of the carb neck, right at the top of the motor case, if that makes sense. Let it sit for 24 hours and you'll be good to go.
Just remember to use it very sparingly as to much of it may cause it to get inside the engine and gum up some other things.