It can be used as a fumigant, in the same way propane is used. It has a higher autoignition temp, so you can get away with higher compression and more timing before preignition. I think you need more to get the same amount of power, though...
Now, if you wanted to totally convert it to natural gas (like the gas 5.9 liters that come from the factory), then you're looking at spark plugs, a distributer, and low-comp pistons.
What does Cummins use to throttle natural gas based on boost pressure?