Can an automatic gearbox improve engine transmission for an ultralight? I am working on my dream to build and fly an ultralight aircraft so I wouldn't like to know the following:


Does adding a motorcycle automatic gear box to an  engine improve on the strength for an ultralight aircraft?  The engine in question is a 4 stroke, OHV, single cylinder, 440cc gasoline engine with a TCI ignition.  It produces 18hp at 3600 RPM,  torque 3.0 kgm/2500 RPM (21.7 lb ft), ignition TCI, gross weight 37 kg.
What should be the tail, flaps and wing measurement proportions for such engine?


Thank you
A gearbox is not a useful addition. Some aircraft have geared propellers, but that's a single, fixed gear. 
The engine has to be able to turn the propeller when it's in the highest gear you'll use, so that determines how much power is required. 

In a car or motorcycle, the lower gears are used to match engine speed to wheel speed, allowing the vehicle to drive at lower speeds than 'idle rpm in top gear' without slipping the clutch or the wheels. 

In an aircraft, that function is not needed: a propeller can slip freely.