There is usually a Citroen 2CV on E-bay. It has a 550 cc. 2 cylinder engine.
Probably the most ecological friendly cars would be the Toyota Prius or that Honda Insight 2-seater with the hybrid gas/electric engine.
The VW 1.9 TDI barely throws out any soot, gets 45+ mpg and they say they last forever.
You could run it on vegetable oil and have an exhaust that smelled like french fries, or for a special treat, burn peanut oil.
Add a teensy bit of Hershey's syrup for a Snicker's-like exhaust.
Semibodacious Transmogrifications a Specialty
1990 300D 2.5 Turbo sedan 171K (Rudolf)
1985 300D Turbo TD Wagon 219K (Remuda)
"Time flies like and arrow, yet fruit flies like a banana"