Alot of the newer bikes these days have over sized tubing that doesnt allow some motors to mount to the front tube. You can make a front motor mount or you can buy one on one of the sites on the side of the forum.They will cost your around 20 to 30 bucks or you can make one yourself which will cost you less than 5 bucks.
I have a huffy cranbrook that I used for my first build and It makes a great Motorized Bike. I wish I had a few extra bucks to spend on a better bike but I didnt at the time.
I believe Pirate Cycles has an engine that will fit the cranbrook with having to pay or make a front motor mount. I could be wrong though. If I am someone correct me.