While not an exact science, you can get pretty close by just looking at the recommended props and seeing what RPM it will turn them. The tachometer readings forum is helpful for that since it gives real world data instead of the wishful thinking of the factory specs. Saitos are more powerful than most other 4 strokes in a given size, so I'd expect the 17cc to fly the plane fine. Of course, that's not all of it. The gasser is going to make its power at a lower RPM, therefore will need a higher pitch prop. That means it will accelerate differently and likely wind up with a different top end speed.