You have two options. You can keep it simple by choosing a gear combo and shortening the chain around that combo, therefore ghetto SS. Or you can take the money route with a new crank, BB adapter + BB(if you want to use a three piece crank) and new freewheel. With the second option you will have to follow Sheldon Brown's advice for respacing and dishing the wheel. Your bike takes an ashatabula crank and those are available if you want to keep to the original look of the bike.
Last edited by hairnet; 08-04-14 at 10:37 AM.