I found this quoted on Sierra club...the gist of it was a two-stroke personal watercraft run for 7 hours emitted the same levels of pollution as a car over 100,000 miles.
While this seems like a fairly large exaggeration, there is no doubt that simple 2 strokes you see around on lawnmowers, scooters, etc, emit far more emissions than the average car due to lack of an exhaust treatment system.
Another site said one of these engines running emits the same emissions as 50 cars.... Sounds a lot more reasonable.