Mahmood: I don't think you can control fuel flow fast enough to maintain stability and control with a needle valve. there are multi stage regulators that could be controled with a linear actuator of reasonable size but again the response time may be to long for effective control.
This is the turbine that your friend is building as a reasearch project isn't it? If so he should design and build the fuel control system leaving you with a lever to control. keep us posted Terry