your on the right track with the single overhead cam,you are two cylinders short technically,but stay here and you will glean some info that will apply just as much to the twin as the fours,that looks like a tidy all original bike!if its cheap enough,have you researched the going price on them?as posted make sure you ride it for more than five minutes when you check it out,,im sure theres a few bits on it common to our older fours.
