I put a 200x cam, (ported) head and carb on a stock rebuild 200m bottom end for my ATC/ATV project, it runs just fine. The 200x cam has a little more lift and duration than most of the other 200cc Honda motors, it will increase the airflow through the motor as long as there isin't a restriction somewhere else (carb, airbox, exhaust, etc). A cam from an XR200r is even hotter, but you've got to have the port size to handle the increased airflow.
you can swap any of the 200cc atc, xr/xl200's cylinder on up to heads and valve covers from 80-83 and 86-02 xr200 or 84-85 xl200r. reason i love these motors cuz everything uses them!