I LOVE THE 100CC ENGINES!!! More than enough power for a bike, if you do it right you can get 8 ponies out of it. I prefer 4-5 ponies, ot's a bike frame, to many ponies will eat the frame. lol. I had though of putting on one of my utiliy engines, till I tried the 100cc steel sleeve. Now it's about all I run anymore. They last forever, produce a good amount of torque and ponies, plus they are under $200 bucks and easier to mount up than a 4 stroke, and lighter, and more powerful.
Only down side, they don't get as good as gas mileage, but close, as a 4 stroke and you have to mix oil.
Oh and they look way cooler, more motorcyclish than a 4 stroke.