What is the heat sink in an internal combustion engine?

The spark plug is not a significant heat source. Imagine a cube in an incline barely holding. A little push and the cube goes tumbling down. The spark plug gives the little push. The chemical energy in the fuel + oxygen system will become thermal energy. The sink is everything else. Imagine that you are driving in hell. The engine is at the same temperature as the combusting fuel and the pressure of the exploding fuel will be comparable to the ambient pressure. The exploding fuel cannot expand. The engine does not work.

The spark plug merely initiates the chemical reaction of gasoline combining with oxygen. This reaction is what produces the heat. The combustion inside of the engine is typically $800-1200 ^\circ \text{C}$. The surrounding environment is the heat sink, whatever the outside temperature is. Some small engines are cooled directly by the air with fins, and most large engines have a liquid cooling system with a radiator. Either way, all of the heat produced by the engine finds it's way to the air outside either through cooling fins, a radiator, or the exhaust gases.

The burning fuel/air mixture generates a lot of heat, which greatly increases the temperature and pressure inside the cylinder. The high pressure gases then push the piston down, providing expansion work that turns the crankshaft. Some of the heat from this process transfers into the water jacket of the engine, where circulating water (from the water pump) flows to the radiator and expels its heat into the ambient air (the environment). The rest of the heat exits the cylinder when the exhaust valve opens, and it travels through the exhaust manifold and exits out the tail pipe.