I am sorely sticking my neck out in an area I only know because I thought this was an interesting question and went looking for an an answer. Therefore, I am hesitant to disagree with Brian, but this does not appear to match pictures I found of C11s, such as the one attached. The gearbox is clearly different and there is just one downtube. I do agree that it certainly appears to be a two-cycle engine of some variety, but I couldn't nail down which one, without making it a career. I think this bike must be built on a A7 frame, but you could tell that in an instant with the number off the headstock.
Richard L.