This guy has got available some 6 volt micro-motors which open up some really wild avenues. How about a skip chassis loco? All the works can be below the frame top, with a single cylinder petrol engine on top. You may even be able to disguise the motor as the air-cooled engine itself. A friend of mine did such an animal in 009!
http://www.nigellawton009.com/6V6mmx10m ... otors.html