If you are talking about a generator with a remote rad, you could be changing the air every minute in the room.
As others have mentioned get the combustion air requirement, the heat off of the block, the heat off of the alternator and the heat picked up through insulated exhaust to determine your air flow.
If it is a gen with a rad and a cowling outside for the exhaust, the air moving for the rad is going to be limited to 0.5" static per NFPA110,
The genset is from kohlerpower. Thanks for your replies. Abbynormal, we dont have to re-invent the wheel right? I hink thats is why there are more experienced engrs and rookies like me...