Hello Harry, - I must say that's a nice job you have done with adapting a B&S engine into that mountainbike frame. I was looking at your Villiers powered bicycle last night and was very much impressed with it. Plainly you have a nack for building very interesting motored bicycles
Yes a Chinagirl can go into a Mixte frame, but it's a tight fit to do it. The final result is worth it though and to my mind the engine looks like it belongs in the frame. With the frame tubes so close around the carb and the cylinder barrel there is also a measure of protection for these components as well.
I haven't made up my mind about a gas tank as yet and may just use the standard tank to begin with until I find something more suitable. Ultimately I would like to make this bicycle as good looking as I can make it, but with a subtle patina of age about it to match the Rudge cycle parts I'm using.