I would say on top of one (or both, for balanced lack of cooling to the engine) of the cylinders.
Because it is an air cooled engine, it might not be real good for the engine.
Definitely NOT near the exhaust port, either above nor below. I know when you cook in your car, you generally just double or triple wrap the meal in aluminum foil and shove it between the block and the exhaust manifold - but that was a long time ago when you could get to those areas. Nowadays there are shrouds and stuff in the way.