I just bought a CNG-powered bus and did a lot of research so I can probably add a bit here-
For the most part, all buses I saw were purchased as-new with CNG and all were engineered systems.
(meaning the engine manufacturer sold the system, versus someone else doing a conversion)
It appears to be very easy to convert a gas motor to run on it.
My own is an 8.1L made by John Deere which normally is a diesel used in tractors etc.
I believe all JD did was replace the glow plugs with spark plugs, and maybe alter the pistons, and of course change up the software.
I'm sure it *could* be converted back- but why bother? It's a unique system that has both its benefits and drawbacks.
Advice: CNG is relatively new even for established engine makers but there are many systems in use today, so if you see a bus that runs on it chances are there's some sort of history around about how well the setup works.
Clean burning and thus very easy on engines and oil
(mine has 460,000 miles, doesn't puff any smoke at all, and it feels like all 250hp is still there)
Ultra cheap fuel: (at 1/3 the price of diesel, it's like I get 27mpg!)
Fuel is either plentiful or non-existant depending on where you live: http://www.cngprices.com/
The tanks take up a lot of room- my bus has no underbody storage because of all the tanks and holds only 50GGEs (like 50 liquid gallons of gas)
The tanks themselves must be inspected every 3 years and have an expiration date of 15 years after they were built.
(my understanding is that there's no actual reason that the tanks have to be destroyed after 15 years, but if you are going to use the vehicle commercially you probably will get in trouble for having expired tanks- and replacement tanks are $$$$)
If you want to spend some money, you can get a compressor for your home that will fill the bus up overnight.
Driving around in a 3600psi bomb waiting to go off? No thanks!
The tanks are over-engineered beyond belief and CNG appears to be about as safe as diesel, and *far* safer than gasoline: