The smell is from unburned hydrocarbons (unburned fuel), old engines do emit more carbon monoxide and oxides of nitrogen (NOx) but neither of those have a smell. Modern cars with catalytic converters absorb a lot of any unburned fuel in the exhaust which is why they smell a lot less noxious when running, they also have EFI and better combustion in the engine to reduce unburned gas ending up in the exhaust in the first place.
The old gas burning in an old car smells worse because it's not burning as completely (more unburned fuel in the exhaust) and there are other weird chemicals that form in old gas to give it that odd smell. A new car with EFI will self-tune to compensate and the cat will capture most of the nasty-smelling stuff.
