Sorry, but if you think 3-400W motors are too heavy for your machine then you have a problem with the structure of your machine. I can not come up with anything lighter than a 300W DC motor but those are a bit too weak in my opinion. You'll be surprised how hard PCBs and copper clads are. If you have a too weak spindle you will not be able to keep up any serious milling speed and drilling the PCB will be a big struggle for the motor. In my first CNC I had a 400W DC motor and it was possible to use that, but was not a very good solution. Now I have a 1.5kW 3 phase brushless high speed spindle and it is like day and night. Can easily work with PCB, using fast feed rates and the results are considerably better than before. I doubt that a quadrocopter motor is strong enough. The best I could think of which is light but extremely noisy is a small Dremel tool, but I would not recommend using those in a CNC mill, the play is just too much for any serious accuracy.