Could you just take a geared setup and extend the shaft? The bulk of the motor/gearing would be farther back into the bulk of the plane. Doesn't seem like the shaft would need to be so long to cause any problems.
Otherwise to answer your question, you have this little guy weighs under 2 grams but might not have enough power.
Then there are these motors. They are available in a wide range of gear ratios and weigh about 10 grams.