Lets face it CO from the exhaust would probably include all the other horrible stuff that stinks.
Even a miss placed crank case vent put fumes in the cockpit that made me feel ill.
Sure, crankcase fumes are yucky and potentially poisonous. If they're coming into the cockpit, you'll likely notice. If you're engine is running with decent pistons and rings crankcase fumes are not the product of combustion. A crankcase vent would have the be very noticeably out of place to put fumes into the cockpit - it is not at all connected to the cabin heat/air system.
A cracked muffler may be directly in the cabin heat system. A very cracked muffler may put noticeable exhaust into the cabin. A slightly cracked muffler may put a hazardous amount of CO into the cabin, without any noticeable exhaust smell. I have experienced this personally. It is not safe to assume that you will detect hazardous amounts of CO by smelling exhaust.