mpg will never be the same as I believe the calorific value of the fuel is lower and you are also burning it very inefficiently by maintaining the ability to run petrol as well (low CR, retarded ignition etc.) Ive often been curious what the results would be if you built and mapped an engine properly for lpg only.

As for the diy option, what exactly can you get away with? Id imagine insurance companies would demand a corgi or other certificate for installations?