I'm not sure I follow you...
You live up a hill.
You have a trike.
You want to ride down in the valley.
You're thinking about adding a gas engine to your trike, to help with the climb back up to where you live.
Correct?

Thing with combustion engines is that they don't scale down that well WRT fuel consumption, noise and emissions.
An engine small enough to only add a little power is hopelessly inefficient. And an engine big enough to be a decent engine is really too powerful to work well with an otherwise stock bicycle. By the time you have a bicycle that's beefy enough to do well with a sensibly sized motor, well, then you have a fairly sucky moped...
If your trike has a single front wheel, consider getting a front wheel with an electric motor in it.