Izzy, the lifetime of engines on NG was explained a little differently to me by people who converted engines to run on NG. NG is much cleaner than gas or propane. So water cooled engine life is almost double what you can get out of a engine on gasoline, and when you get to the time of rebuild the inside of the engine is very clean since it has much less abrasive carbon in the fuel, which is also why NG has a lower energy density. One of my customers used CNG in delivery trucks and routinely hit 400,000 mile before rebuilds were necessary, and these engine retained the ability to run on gasoline. When the high mile engines were torn down the cylinder bores were usually still within manufactures tolerances and didn't need to be re-bored. The other customer made NG powered air conditioners, back up cooling for hospitals and such. The V8 powered AC's were rated for tear down at 40,000 to 60,000 hours.

The problem with air cooled engines as it was explained to me is in the mechanics of the fuel not going though a state change to help cool the engine. Air cooled engines running on gasoline are usually run fuel rich, as the gasoline goes from being a liquid to a gas it helps cool the engine. Running an engine on NG there is NO cooling effect from the fuel, so running an engine in the Florida or California heat on a fuel that is not helping to cool the engine will usually toast it.