Im assuming that the conversion is of a diesel engine from straight diesel fuel to lean-mixture natural gas ignited by pilot charge of diesel oil. While thats highly economical based on the low price of natural gas, it also is a very efficient process thermodynamically. And since the combustion of carbureted air is smoother than the combustion of injected fuel oil, I suppose there is the potential for maintenance savings because of the lack of diesel knock. Are there other maintenance savings?
Then throw government/EPA regulation in on the diesel fuel and do the math.
I know asphalt plants here that switched to natural gas from burning fuel oil saved enough in maintenance costs in the first year or so to negate the cost of the conversion. That's external combustion, but their burners lasted way longer and didn't need constant upkeep.