Good question Kev
Getting Motorised bikes on the road in Oz has been an interest of mine for several years.
I think that 2-strokes have a lot to offer because of light weight. The lighter the vehicle weight, the less power (pedal + motor) to push around. You would think that the environmentalists would jump at it, but the bogey they bring up is dirty 2-strokes.
Sure, 4-strokes are also MBs and they are substantially illegal in Oz as well. 4-stroke bikes tend to be heavier and less likely to be readily operable as a pedal cycle.
In my view, the holy grail MB is; fully operable as pedal cycle, 1.6 kW (2HP) motor, vehicle (bike + engine) weight less than 25 kg, 50 km/h (30mph) top speed under power.
At this stage, there is no bike-engine market potential in Oz until the road regulations are changed. So far as 2-strokes, I suspect that means clean strokes, although it would take 20+ years for dirty 2-strokes to ever be a problem in Oz.