This question has a lot of variables that need to be stated.
What heat sources are located in this room?
Exhaust System; how much of the exhaust system is exposed in the room. An estimate of heat gain from the exhaust system can be made from exhaust gas temperature and heat transfer coefficient for exhaust system and amount of exposed system. Exhaust temperature depends on the type of engine, efficiency, and loading.
Engine Block; you could assume 200 degF surface temperature and determine the surface area of block and cooling system hoses. Estimated heat transfer coefficients are available in heat transfer books or engineering manuals.
Radiator; what removes the excess heat from the water jacket? The radiator is located where?
Will the output power be used in this room? Will this be a generator or a water pump or what? What is the efficiency of the power transfer?
Ventilation will need to be provided depending on the above factors.
I would get the engineering data for this engine. If not available, look for generator set or marine engine with similar characteristics and get the engineering data for it. Then you could estimate heat loads and ventilation requirements from there.
