if the screw is stripped i would just use a tiny amount of epoxy or jb weld, i actually rethreaded a idle screw with epoxy and it still turns great (put oil on the screw before i put epoxy so it didnt stick)

as for the prop issue, ive noticed that reliablility when turning a big prop is not their, while it may work every now and again the engine is much more reliable with a smaller prop, not to menton you use alot less fuel and get a ton more power