I have an astro hog with an os 91 four stroke in it and it flys with great authority. Probably would fly with less, but I would question whether a 40 would be adaquate,especialy if this is a plain bearing engine. They don't as a rule have as much power as a bb engine. I would think a ball bearing 46 or larger[52 to 70 four stroke or 60 two st] would have much more authority.