The Mega 16/15/X series of motors will strap right in place of the stock motor, as they're the exact same size.

If you're going to hop your Zagi up with a super-duper brushless motor, you're going to need to reinforce the wing, or you'll have catastrophic "Zagi flap." The wing will literally start flapping like a bird's, and it will break in half. Zagis are not rigid enough in their stock form to handle speeds much higher than what they can achieve in their stock form. In fact, a high-speed full-throttle dive will get the plane flapping.