Basically, the newer 2 stroke * diesel engines are much improved, but there's a huge number of older 2 stroke gasoline engines which can even be significantly improved with a slight redesign. Some kids from Denmark were in the news recently about the latter. Although the noise of 2 stroke engines can be much reduced with adequate mufflers, one local chap has a retrofitted bicycle with a gasoline 2 stroke engine which he rides around town, but it's a cheap DIY retrofit lacking in some necessary details.
The noise is so loud one can hear the echoes of him passing a mile away. A group of those bikes would awaken the dead.
*
http://www.pivotalengine.com - A 2 stroke REDESIGN for scooters, motorcycles, or cars, but possibly could be adapted to smaller frames like bicycles.