Most 2 strokes put out today have a bit more power than those of 1967. I suspect a .46 would be just fine. The smaller biplanes fly a bit squirely at times and can be a bit challenging to fly. I'd start with a .46 2 stoke and go from there. But weight and wing loading is a major factor in selecting the engine size. I'd suguest weighing your airframe and see what you have to work with. Do you show an wing area?