I got mine from Horizon Hobby for $139. You can probably get a 700 in there. The existing 550 makes it a little difficult to get a standard stick batt pack in there, so a 700 would make it even harder. The existing motor coupler would have to be replaced to handle the larger 700 shaft (5mm instead of 3.2mm), and I'm sure the motor mount would have to be modified.

I figure I'll probably try a 600 first before trying to cram a 700 in there...