You could get a golden eagle kit and move/lean it back behind the seat rack you have, and if you wanna 2X the pep in the engine get another one to mount on the front wheel. The small 4 stroke engines enough HP for 1 person but if your trying to carry a passenger you might want to double your HP with a second engine unless you are willing to compensate with peddling.
2 weeks late but anyway - your best bet would be a vintage motor that fits under the bottom frame tube, next to the bottom bracket. Either that or another type of vintage motor called a Winged Wheel or Cyclemaster - these are hub engines so you'd need the back wheel rebuilding around it.
Got a new bike yesterday for about $40. Any chance of fitting an engine? I don't think so because the top tube is so low, but I want to get a second opinion.