Try the Honda dealer. They are surprisingly good at providing parts for ancient bikes. Also, you can look at their diagrams and compare it with what you have to see exactly what you are missing.
Alternatly, look at the locak bike classified ads. These 200 cc engines were made for more than 20 years in pretty much identical form. Possibly somerone is parting one out. If so, get the entire carb and throttle cable and put it on.
BTW I seem to remember in the early '80's they made a 4 valve XR 200 for a couple of years (not good for parts) but they went back to the 2 valve engine like yours for many years afterwards.