From what I saw with a fast search the vent nipple on the back of the crankcase (I assume that is what you are asking about) feeds back to the fuel tank to bleed out all the oil sitting in the crank case to mix back in with remaining fuel Other wise it would build up and put back pressure inside the motor and eventually fill the entire motor as it runs.
Thanks! thats was exactly what i eas vondering about. Im going to have it on a (You Can do 3D 60 plane)
If you are looking for just the bullet style spinner I saw a picture that said its 1.25 inch / 32 mm spinner. If you are looking for a cone type that is more dependent on what prop you use.