If you think it's exhaust, don't fly it. Yes, there is a list of symptoms for CO poisoning, but it's also known as a silent killer because it can sneak up on you and impair your judgment and skills.
Carbon monoxide by itself is odorless, so the exhaust fumes you smell are unburnt gas or perhaps oil.
Another possibility (but not one I'd bet my life on) is that some oil or fuel was spilled inside the plane for whatever reason. Could you smell fumes BEFORE you started the engine, or did it only occur in flight? Did you notice the fumes after starting the engine but before you took off?
Did someone spill oil on the engine, and somehow the fumes from hot oil got into the cockpit? (As a student pilot, I once had what seemed like a lot of oil blow onto the windscreen as I started to descend for a landing. What was going to be a touch and go turned into a full stop. Once on the ground, I realized someone had accidentally poured oil on the engine rather than in it...)
Whatever the cause, you shouldn't be smelling fumes, and shouldn't fly that plane until you know what is going on.