I've seen many videos of small turbojet engines (small enough to hold in your hands) that produce 30-40 pounds of force. If you mounted a couple of these on a small craft, that wouldn't even support human + gas, and especially wouldn't support a fuselage or any other casing.
What size is efficient with gas but still produces a lot more than 40 pounds of thrust?
An extra question you don't need to answer: what is an effective way of tuning fin direction on the exhaust end of one of these engines? Basically, how can I twist a knob and make the fins tighten or twist in a direction I want the thrust to move in?