I would recommend a multispeed bike if you are riding off road, as that will make your single speed motor more fun at the top end of the motor range, where you will want to shift into high gear on your bicicle to assist the motor in getting up to its max rpm's and your total max speed. otherwise, your motor which may be capable of 7800 rpms will only get up to 6K or so without pedal assist to overcome your rear wheel enertia. By the way, do you know how fast 40mph off road on a mountail bike is? its fast.
in regards to your other question abou the mount. If your friends dad can fab a center mount that would be the way to go. and its nice to have the motor in the center of the frame for handling.