An 18cc engine is about 1.10 cubic inches. The conversion formula is: CC's X .0610237= CI. Your 18cc weedeater engine would be pretty heavy for a 40 size plane. I have done a few 16cc engines but not good for much more than a floater plane like a Cub. I have also done some 21cc Echo engines and had them on .60 size planes. A Seagull Models Edge, picture attached. It flew but was lacking for any type of 3-D. Sport flying was fine. There is no chart for props that I know of anyway. Gas engines don't turn RPM like a glow motor but will turn a bigger prop. I try to prop my engines for about 7200 or so and go from there depending on how she flies. Some engines are ported and port timed better than others so they will run better. Echo, Stihl, Husqvarna are easy to get and run very well in my time and experience. Hope this helps.