small brushed motors thrust
I recommend looking for spare motors for existing quadrotors and back-calculating from here. Here is one example:


Google for "quadcopter dc motor"
First link: http://www.robotshop.com/en/crazyflie-20-mini-quadcopter-motor.html
Corresponding quadrotor: http://www.robotshop.com/en/crazyflie-20-mini-quadcopter.html
Notice motor weight: 2.7 gramm and whole copter: 27 g + 15g payload
This means total trust is 31g + motors' weight


So these motors do not work. Search for the next motor and repeat the exercise. Keep the notes in spreadsheet or a text file. If you checked 30 motors and none of them worked it is time to give up.