A cheap 50cc kit from eBay: $140AUD+shipping
DIY kit: $89+shipping
Predetuned 50cc kit from ZBox: $375+shipping
Predetuned 50cc kit from RSE: $425+shipping
I think there's a good reason why I'll be getting the DIY kit.
However I'd be very interested in how well the engine runs after detuning. Does it have all the low end torque as advertised?