I use heat to loosen old carbs and engines. A heat gun always does the job. RBACONS is right about the carb lever. Tighten down the throttle arm, then use heat. Don't force it; wait for the heat to work, and try it gently until it does. Then put it some light oil.
If you get into the hobby, then you might want to have the other Saito fixed, so I would put it aside and keep it. Then if you want to use it later you can order a part and have at it, or send it in to Horizon and have them fix it. It will cost, but it was free to begin with and you will have a good engine after they go over it.
Jim