Cometravi,

To calculate approximate correct carb venturi size:
D = K x Square Root(C x n)

D = carb diameter, in millimeters
K = constant between 0.65 and 0.85 (determine smallest & largest D for engine)
C = cylinder displacement, in cubic centimeters
n = RPM at peak power/1000 (be realist, dreaming only will make you try carbs far
bigger than the correct, with results below your needs)

For a Ryobi 31cc running @ 8000

31*8=248
15.75*.7=11.02 carb dia in mm

From your specs 35*7=245 .....> yeields 10.17 to 13.30 mm diameter

Hope this helps. It is just something I saved from another post, forgot who's.
On the engine: What size /shape is the intake port? (May give you an idea of sizing of alternate carb)