If you wanted to convert a car to run on a gaseous fuel, compressed natural gas (CNG) or liquid propane (LP) would be your best bet. CNG and LP are proven technologies (whole fleets of vehicles run on them in some cities), and you will at least be able to find a few refueling stations. CNG and LP are also a LOT easier to store and handle, as well as cheaper than hydrogen.
Another problem with hydrogen currently is that most of it is made through electrolysis (someone correct me if I’m wrong), and that combined with the energy required to compress and chill it for storage means that it takes more energy to produce it than you get from it. So it ends up being only as green as the power generation technology that provides the energy for its production, and wasteful too.