Well fuel costs is almost same with a little edge over milage. For a 1000cc it must do 13km / liter. And it's 75 rs / liter in Islamabad.
There are no as such disadvantages of lpg as i personally belive that butane is better than petrol, so it will be good for engine if not bad.
No alterations as such except some piping to keep lpg liquefied. Shouldn't cost more than 1k
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