What can cause a 1981 Yamaha Virago 750cc to fail to spark on one plug?

If the OTHER plug has spark, you're left with the parts specific to the offending plug: it could be one half of your TCI (the transistor control ignition unit), the corresponding ignition coil, the spark plug itself, or any of the wiring connecting the above. That includes of course the thick spark cable from the coil to the spark cap.

This is a rather fortunate situation, however, since if one plug is working, you can switch components between the two sides to find out which is carrying the fault with it to the other side when swapped. One would test all that with both plugs OUT OF THE CYLINDERS, of course, since reversing ignition sequence with spark plugs in-cylinder is not exactly a genius idea.