These are all the rage on the endless sphere lately:
http://www.gngebike.com/36v350w-48v400w-gen2
If you are into fabricating and adapting your own design, there are many options for you, from using RC airplane motors, to building your own motor. The hub motors are the easiest to adapt, and even work good outside of the hub like the military bike posted prior. One huge concern is going to be the length of the motor, as a long motor will stick out and get in your way of pedaling and such. If you knew what the bike looked like, we could probably give better advice.
http://www.gngebike.com/36v350w-48v400w-gen2
If you are into fabricating and adapting your own design, there are many options for you, from using RC airplane motors, to building your own motor. The hub motors are the easiest to adapt, and even work good outside of the hub like the military bike posted prior. One huge concern is going to be the length of the motor, as a long motor will stick out and get in your way of pedaling and such. If you knew what the bike looked like, we could probably give better advice.