petrol engines need little modification, what is def necessary is another tank for LPG. different fuel supply system is required, but if original is kept in place either petrol or LPG can be used as needed. propane is a liquid under pressure and much denser than gaseous compressed methane, and not cryogenic like LNG. diesel engines can also be converted, but it gets harder and requires either small amount of normal diesel used or installation of spark plugs (it’s still diesel cycle)
propane is like 3/4 density of petrol and gram per gram carries more energy (propane 0.58g/ml, petrol 0.7ish g/ml) it’s slightly greener because it contains more hydrogen so more energy per carbon emissions
LNG is cryogenic, has even lower density (0.41 to 0.5 g/ml depending on temperature) and CNG is less dense still depending on pressure