I would add to ray's recommendation that the selected motor should have a Kv no greater than 1000 to turn the big folder props. i have a butterfly and this thread is sparking some interest in me to go electric with it as the curren tired OS 15 just about gets it aloft- where it hangs on thermals just nice.
I think I would look at extending the nose about 2 to 3 " to get motor and battery forward and CG right - I hate dead weight.

I gave up lawn chair flying - tooooo relaxing !!!

john