I can only comment that from the look of the forks, it's an 80 or newer model....I know, not much help. Can you find ANY identifying marks like a serial number?
What did the sticker used to say? It looks a lot like an old AMF "moped" from the 70's adapted to a tandem bike.
My brother had a AMF moped with a rear mounted engine like that on a really crappy frame. He bought it new for 99.99 in 1979 or 1980.