This is by far one of my favorite DIY projects.  I have yet to do this, but I think that it would be good to know how to do in a pinch, or to just keep in the garage in the event you needed to pull it out and utilize it.  I have seen multiple versions of this plan online but came across this one today and figured I would share it with you:  http://goo.gl/XEx4

Here is a PDF I have found as well:  Build a generator from a lawn mower motor

Pics of a Lawnmower Generator found here: http://goo.gl/Nc7t