Here is another informative link. From it - While both propane and natural gas have higher octane ratings than gasoline, litre for litre they have less energy as they are less dense. This apparently can be offset by increasing the compression ratio of the engine by replacing the pistons and/or the heads depending on engine design.
http://en.wikipedia.org/wiki/Gasoline#Energy_content
This company sells a kit for diesels.
http://www.omnitekcorp.com/altfuel.htm