I think an engine with a belt reduction drive would be ideal. run a large prop at a low rpm to gain most thrust. The strongest engine to meet these rules is a MB Profi 40. you can have these set up from MB to run on higher nitro. Just a guess here but on a belt drive and say a 20x10 prop at 6000 rpm or so should give around 18 pounds of thrust. that would fly a 30 pound plane.