I used to build paper airplanes a lot as a kid. I once tried to graft one of the rubber band propellers from those little balsa wood planes you could get at the store onto the paper planes with no success. I was never able to get the balance right and the planes always crashed. A cool new gadget called Power Up has broken cover that will give you an electric engine for your paper airplanes.
The Power Up is a toy made by Tailor Toys and it has a little electric motor on the back, a long stick, and a propeller on the back. The capacitor is actually on the front of the paper plane and the propeller is on the back in a pusher layout. The capacitor gets power from three AA batteries in a little charge box.
A 20-second charge is enough power for 90 seconds of flight. It seems that I would have the same issue of balance with any paper plane I made for this motor. You can buy the Power Up for $17.49 on Amazon and it looks really cool. See the thing in action on the video below.