I'm going to use a cute little outrunner with a 10-4.7 prop (2408-21T). I have that same motor in my "Butterfly" ARF and its amazing how much power that little motor has!
Hobbycity Online R/C Hobby Store : TowerPro Brushless Outrunner 2408-21 $7.95
Its pretty cheap, so I'll try it first and then go from there and try something else.
I think your design uses the old 400 geared motor too, like mine. I'm designing a unique motor mount to slide into the gap, where that old gear motor would mount to.
(I'll see if I can get some pics)
Jan