Most of the formerly diesel busses in LA are now running on natural gas (methane). If I were to pick an ideal alternate fuel, that would top the list. I have no details on the conversion. My gut feeling is that it is very difficult. Gas is "dry" compared with fuel oil, meaning it has no lubricity. The big issue is cetane number, simply stated, a rating of how well a fuel auto ignites in a compression ignition engine.