Guys,
Take it from an engineer, you CANNOT convert a petro/gas/diesel engine to utilize hydrogen gas technology. These advertisements and claims are completely false and a rip-off.
In a true hydrogen fuel cell vehicle setup, you purchase compressed hydrogen gas (which is produced by splitting water with electricity into oxygen and hydrogen gases, typically in an off-site or roadside facility using solar or wind energy). A completely electric vehicle is then powered by recombining the hydrogen gas and oxygen from the surrounding air inside a "reactor" to reverse the process and make water. The water goes out the tailpipe, and the by-product of the reaction is electricity which is used to power the drive motors and systems on the vehicle.
A true hydrogen vehicle does NOT burn hydrogen (in fact, there's not combustion involved whatsoever), it simply utilizes hydrogen gas as a carrier to produce electricity in the end. Believe me, it's the future and will solve our global energy crisis if implemented correctly. Check out Denmark and the rest of the Scandinavian countries, as they've been taking advantage of hydrogen gas reactors for over 80 yrs now. I believe one country has now implemented a "Hydrogen Highway", which only allows hydrogen fuel cell vehicles to travel on it. Problem right now is range, they can only travel roughly 250 mls between gas charges. That's bound to increase as the research and development continues, however.
Hope this helps clarify things...