Virtually nothing is interchangeable with any of the 500s. In the 650 models, the engine is the same, that's about it. The CX650C was common enough in the USA so parts are generally available, except for the left side cover. If yours has one, treat it lovingly. I have some 650C parts for sale, email me when you need something. I believe others have some 650C parts as well. I wouldn't sweat the parts availability, but forgo the 500 unless it interests you as a whole bike.
motorcycleguy DOT rich AT gmail DOT com