I have
a cheap clamp on mirror. It may be the cheapest mirror you can buy, and it moves around a lot, so it takes regular fiddling. If you're looking to satisfy an inspector by demonstrating that you have mirrors, it's amazing. If you're looking for a solid and reliable rear view mirror, there are probably better options. I haven't tried them, I just adjust my mirror a lot.
You can also buy a
mount that bolts onto the steering head to open up your options for other mirrors that stick up like antennae. I have one of these, but no mirror on it. There's not a lot of room between it and the cowl, so if a mirror were bolted to it, the nut might bump up against the cowl. It might be workable, but it's definitely tight.