Designing a hydrogen engine is relatively easy. The quickest thing is to just go with a fuel cell and run directly to electric motors or batteries. If you want a true combustion engine, there are some issues - flame speed is faster than gasoline, hydrogen is so small, the flame can "sneak" past the valves, hydrogen can blow past the piston rings and light off in the crank case, plus a few other quirks.
The big issues are: Cost - call around and price a fuel cell with even 1/4 the power of your gas engine, then team that up with power controllers and electric motors and you'll soon have 10's of thousands of dollars invested in your power plant. Availability of the 'fuel' - call some gas stations and see how many have hydrogen, you won't be able to drive very far from your home base. Availability of the raw material - hydrogen isn't ever found in it's free form. At best it is extracted from natural gas or oil, or (in the lowest efficiency) - water. Storage - If you want to store any appreciable amount you're either looking at ultra high pressure tanks (dangerous), metal hydride (expensive, slow to fill, slow to release, may require external heaters and coolers, etc), low pressure tanks (bulky, low energy density) or cryogenic (requires large energy input to liquefy hydrogen, don't plan on parking your car for a week at the airport and having a full tank when you get back, etc)
Then there is the minor issue that water vapor (result of burning hydrogen in your engine) is itself a greenhouse gas, so if you are willing to put aside the global warming hysteria and pump it directly into the air, that is one option. You could condense it and just let it drip onto the ground - although it would probably be like driving in a permanent rain storm with thousands upon thousands of cars running down the road with a constant drip coming out of the tailpipe. I suppose you could trap it in a tank and dump it at every fueling like dumping an RV toilet, but that doesn't sound like much fun.
Bottom line is, even if we had a perfect hydrogen engine today, hydrogen is pretty far from a perfect motor fuel.