A regular bike computer would do the trick.
If you don't want to insult your noble steed by mounting such a low-tech accessory, a bit of Macgyvering ought to let you tape the magnet to the roller and the pickup and display to the frame of the roller.
Keep in mind that roller/trainer miles don't compare well to road miles. IMO it makes more sense to ride by time and effort rather than a synthetic mileage/speed.