it'd be best to order one that fits that bike. Speedometer cables ends are formed to fit into the wheel and speedo. So, you can't just lop an end to make them fit. A bike's cable may be too thin to drive the moped speedo.. and there'll likely be other difficulties.
i ran with a "bicycle computer" for a long time.. it tracks speed, distance traveled, time, everything else you'd care to know and a lot of things that i found useless..
They are accurate and pretty easy to set up. A magnet attaches to a spoke and a sensor is attached nearby. The main unit is mounted on the handlebars... about $25 will get a half-way decent one.