A starter motor is a bad choice for this application.
As you mentioned yourself, it is not designed for continuous duty, even at low currents. At least the brushes or bearings will wear out quickly.
But there are other issues:
regulating a starter motor is a nightmare, it's simply not designed for it. At the very least, you need tacho feedback to control it.
Also, it is designed to give a short, very high torque burst, and will probably kick you off the bike.