As long as you are not trying to hover on the prop, a 35 cc engine will fly a 25 pound airframe at 45+ MPH. Done properly, mufflers don't add much weight wont rob any power. Your biggest challenge is likely to be weight. It sounds like your payload will be just over half of your all up weight. You are also looking at about 3 pounds of fuel. Hydrogen wont be much advantage over gasoline in a small aircraft. The fuel is lighter but tanks and regulators have weight. There is a program on the web called motocalc that will let you plug in all of your parameters to compute your need for a motor battery combo for electric flight. I suspect that flight duration will be your biggest challenge there.