I've heard that conversion kits to LPG cost in the range of $2k.
I also heard that LPG has a higher knock resistance than regular gasoline.
And while LPG is rated at 110 octane,
Natural gas is rated at 130 octane, which means you could potentially run major boost on that!
And it's cheap, like pennies, meaning, you could run your car at a fraction of the cost of running it with gasoline.
LPG and Natural gas are 2 different things, but they both are gas, and environment friendly. But natural gas has a higher octane, and is cheaper.
At the same MPG, you'd be running at half the price (~$1,37 per comparable gallon of gasoline).
There just aren't many natural gas stations on the roads.
See
this map
Haven't found any information on any conversion kits for the ST.
Lots of other cars do have conversion kits.