How about propane power? It is very clean and moderately quiet and methane or hydrogen is also a more eco friendly alternative.
Extremly small and light engines made for ultra-light aircraft should be extremly well suited for something like a recumbent trike as well. If you had the skill you could rig up a chain or even belt drive to the front wheel and keep pedal power as well. It would be a challenge, but certainly possible and it would be able to deliver some extreme fuel-mileage and power/speed.