Actually from the info I found (which looks fairly reasonable) mopeds are not great, but aren't as bad as are being pictured (certainly not as bad as lawnmowers and such) - the figures I found (check the thread called "mopeds and pollution) seem to suggest that compared to a new car, a moped will put out about 4-5 times the amount of emissions in SOME categories... which isn't great, but hardly the 50-75 times people have claimed.
And when you take into account that they do burn much less fuel on average (gasoline puts all kinds of nasty emissions out during the refining process, waste gases are burned into the atmosphere, and petroleum is burned to heat the crude during the refining process as well), and that they take a hue amount less plastics and steel (steel refining puts sulfur, N2O, etc into the air... per ton of steel you might get something more than the total WEIGHT of your moped in highly toxic emissions)
Plus this is presuming you are driving a modern car.. anyone driving a car made before the early eighties will be putting out 4 times the nasty emissions of any moped.
Given that the average moped takes 1.1 riders, and the average car takes 1.7 riders (most cars carrying only one passenger) - they don't appear to be quite as bad as you would expect. In fact, when you look at the resources consumed to produce a car, how much of the car remains unrecycled/toxic after the lifespan of the car, how mopeds take up less space in traffic which reduces traffic congestion and traffic jams (which result in hundreds of cars sitting there idling and putting out emissions without even moving), do virtually no road damage, and generally use and leak lesser amounts of things like ATF/hydraulic fluid, air conditioning coolants, engine coolants (antifreeze added to the radiator), crankcase oil, etc etc etc.... its not nearly so cut and dried as people make out.
This being said, the ideal situation no doubt is to design a better moped - already apparently underway in some countries (india has a push on for this) - to add catalytic converters which can reduce emissions up to 38 percent without signifigantly affecting power, or look for alternative power sources (electric isn't there yet in range, power, or expense... mostly the problem being the batteries I suspect. But this could change when fuel cell technology matures in the hopefully very short term future)
Just thought I'd chip in on this thread.