If you use Google, you can find countless sites that discuss this very thing. No doubt hundreds if not thousands of people have done it. However, from what I've read, it may not be such a great idea. Weed whacker motors are very loud and produce smelly fumes. That's because they're usually 2-strokes. Putting it on the front wheel makes it even worse because it's closer to your face and the fumes are being exhausted in front of you.

If you're going to mount a motor on the front wheel, you might want to consider using an electric instead. A motor from a rechargable electric power-tool might work. I met a guy in Seattle that used an electric wheelchair motor with batteries made for a RC race car on a bike.