for the King II the Esly 3900Kv motor drops right in and is a good runner. FYI, the Kv is a voltage constant and denotes how many rpm per volt you get with it. It is not an indication of how much power the motor has.
You will aslso need to add a little support to the motor mount area because it is inherantly week. Extreem has a frame stiffener that works, I invented some braces called the Shizzle mod. There is also a little plate you can make to go under the motor area.
But, consider that any money you spend on it could better be served going towards a Exi450 bare bones kit.