I have a commercially made bike motor, drives the front wheel with a weedwacker engine. The drive is by friction roller, doesn't work well in rain.

Local guy built one very similiar.  Was very difficult to turn with the motor pushing you.  But was great fun anyway.   I told him it wouldn't work but he made the push motor swivel to help with the steering issue.  Ofcourse when he turned the bike steered fine, except the push motor didn't follow the bike and ends up just knocking you over lol!

He moved off to california so I don't know if he ever got all the kinks worked out... but I'm suprised they are not for sale at wal-mart yet..