You can use an automotive coil made for older points ignition systems. If that's too complicated for the guy behind the parts counter, tell him it's for an early 80's Ford lawn and garden tractor with a 16 hp Kohler engine (engine model 341 I think). While bigger then the original, surely you can find some place like the front frame down tube to clamp it too with a hose clamp. It will also much cheaper ($20-$30) but best of all, much more powerful. Gap the spark plug to .035" and you'll find it starts and runs better, especially in cold weather.