Tutorial on the basics of intake design
The angle is important -- no 90° angles for optimum flow, but a modest angle or slight turn doesn’t hurt anything.
Length should be minimal.
Round is best, but sometimes it’s impossible for transitions to reed blocks, ect.. In that case, you want the transitions to be smooth as possible. Some four strokes use a D-port. The flat side on the bottom helps the flow wrap around the inside edge of the valve while the round top feeds the far side.
It is important to not change sizes from venturi to manifold and back. Keep all the sizes the same to provide a more consistent signal to the carb jetting.
Smooth is good, but polishing doesn’t help unless you are running WOT all the time. I use a light stone finish to leave a slight roughness that aids with atomization/suspension.
Don’t forget a low restriction air filter for maximum flow.
Most anything will work on a moped unless you’re trying to set a land speed record. But with big/obvious variations in shapes or dimensions from these standards it can cause problems with getting the jetting right at all RPMs.