I haven't done it myself, but I knew a guy that did it in his garage, and as I understand it, very little engine modification is necessary. Used a Camaro--lost about half the horsepower and the range of the car wasn't what it was, but as I recall, the only major modifications he did was added a storage tank for the hydrogen.
Of course, you do run the problem of "cracking" the hydrogen (as the article puts it) in your garage. Hydrogen can be dangerous stuff, and I don't think your insurance agent would take too kindly to filing a report about how you blew up your garage...
Gasoline burns pretty well too. But then, we've become accustomed to the safety issues involved, and we're used to the risk.
I'm certian that there are ways to make hydrogen "safer". Probably safer than gasoline, which tends to spread all over and flow in wrecks. There are several techniques that might be used with hydrogen. The "explode" component of hydrogen is a problem, but again, I think that can be controlled. Cost and ease-of-use I'm not sure of.