the specimen I have has significant downdraft on exhaust, 29 degrees on the top and 45 degree on the bottom wall of the exhaust manifold chamber. I have not measured it against TDC and piston center, but I am quite sure it is exactly alligned to the top of the combustion chamber.
It is done to aid scavenging effect as well as to bring the muffler mount lower and nearer to cylinder base. The idea is that as the piston moves down on the power stroke, the gas expansion follows the piston movement and achieves its highest expansion speed near the piston surface. As it reaches the exhaust port, it follows its downward and outward trajectory and has a smoother exit through a downward sloping exhaust manifold chamber. It is an excellent design element that increases the volumetric efficiency..
Industrial engines have straight box exhaust chambers. Larger glow engines such as Supertigre have lower surface of the box angled at around 15 degrees but zero degree on the top.