I finished concrete for a short time...in SoCal...in the late 70's. We had a couple trucks that were gas / Natgas equipped. We would switch the fuel to Natgas..on the FRWY..and pray that it would work. It generally did....
I guess my point is...we've had the technology for quite some time.
Using the original gasoline engine and the only change is to feed it with a much less energy dense fuel like they did in the 70 is a joke.
It would be like cutting the gasoline fuel pump to half flow and expecting the same engine performance.
The new applications use an engine design for this lower energy fuel. That is why it cost so much.