21 inch wheels on metric bikes

I have a 74 Yamaha TX650 that I am bobbing. I like the look of 21 inch wheels, and I wonder if anyone has tried a junkyard conversion. Can you adapt an HD wheel to the forks and fab together brakes? What about using a dirt bike wheel from the same manufacturer and era? Any experiences or suggestions?

The maker of the wheel doesn't matter. What does matter is the diameter and length of the axle, the width of the hub, possibility of need of a disc brake spacer(s), consideration of whether you are running 1 or 2 brakes up front, and the possibility of needing to cut wheel spacers. Simple, huh! I just got done doing a custom fab on the front end of my bike. use PVC pipe to do the mock up in place of the real spacers, and once you get them cut to the proper length, then have the real spacers done up using the PVC as models .