I sold my 1977 900SS last year, and wish I had it back.....
Looks like the real deal, post the engine and frame numbers to settle it. As stated before, bottom ends tend to give up if the bike is abused (over-revved or run wide open when cold) and worn gears jump out of gear. Engine is an absolute jewell, but a real bitch to re-assemble, since a lot of shimming is involved, and of course the valve adjustments are not exactly straightforward...ignition another weak point. The stock Ducati Electronica ignition should be chucked, since it goes from none to almost 40 degrees of advance before you even clear the intersection, further, stressing the bottom end...Lucas RITA is one of the best ignitions for this bike I've found, but they are hard to find and many don't like that the reluctor takes up the rear bevel tower, so you can't have one of those neat "gear windows" on the rear tower....The PHM 40s suck a lot of air and the bike is finnicky to get to idle unless carb perfectly balanced and adjusted and timing dead on... Lastly, generator/ignition wires run in hot oil bath and tend to degrade, must be replaced....
All that being said, one of the must visceral and soul lifting bikes you can ever ride, and when one is dialed in, they are like...magic....Good luck, and whatever effort you put in will be returned in spades......