Thanks to all of you for your replies. I put in a Shimano 13-28 7 speed, that should do it. I didn't change the chain. Just can't see how a chain could stretch with such little use, but if I need to change it, no big deal. I got the bike on Craigslist with the idea of keeping it in my trunk and being able to ride it whenever. Actually, though, I've just been riding it on weekends and am very happy with it. I have a road bike that I haven't been on because of the Loop. This thing is just so much more comfortable, with its low stepover to get on and off, its maneuverability and fatter tires for road hazards, storm drains and trails.
For $200 bucks new, I can't figure out why there aren't millions of them on the road.