If you have an electronic rev counter that can be selected to 4, 6 or 8 cylinders you can open them up and see the resistors used to make the switch. Then you can work out the resister value for a 2 cylinder engine and use a that.
I've no idea if it would be safe to use lpg in a 126, or where you would store the gas. The cost of converting may never be worth the saving on such a small engine anyway.
I'm not sure what you were trying to say about electronic ignition. It will make the car run better and therefore be more economical, but as these are not fuel injected cars, the saving is not great as you are stuck with the same old ignition curve.
You can swap carbs, but this is usually done to get more power, not save fuel. The easiest way to save fuel is to drive gently.
