A 20mm projectile is a significant caliber typically associated with heavy machine guns and anti-aircraft weapons. It is highly unlikely that the engine of a Harley-Davidson motorcycle or any typical motorcycle would be able to stop a 20mm projectile.
Motorcycle engines, including those found in Harley-Davidson motorcycles, are designed to withstand the internal pressures, heat, and stresses associated with combustion and power delivery. However, they are not designed to provide ballistic protection against high-velocity projectiles.
A 20mm projectile fired from a machine gun or anti-aircraft weapon would possess an immense amount of energy and velocity. It would easily penetrate through the engine block, which is made of cast metal or aluminum, without significant resistance.
It's important to note that motorcycles, including Harley-Davidson motorcycles, are not intended to be armored vehicles or provide ballistic protection. Their primary focus is on providing transportation and an enjoyable riding experience.