Even if you had a huge gear reduction on that weedeater/stringtrimmer motor, I don't think it'd survive as an ebike drive, unless you wanted to have a very small assist to your pedalling. Without a huge reduction converting the very high speed of those motors to torque, you'll probalby just burn the motor up fairly immediately.
I recommend reading thru some threads in the non-hub motor drive section, on various DIY types of builds, to see what others have done and what it took to run their bike the way they need it to, under the conditions, speeds, accelerations, terrain, winds, etc that they ahve to deal with.
You can also play around with the calculator at kreuzotter.de to figure out how much power it will take to do what you want.
There's also a simulator at http://ebikes.ca/simulator that can help you find a set of components/conditions that will work under your power requirements/etc., though it's probably more info and a steeper learning curve than you need right now.
