It all depends on the aeroplane. A small amount of thrust can fly a large and heavy glider, but would not fly an F-104 of similar weight. So there is no equation that says 13lbs of thrust can fly a plane of X weight. The amount of thrust required to maintain level flight at a particular speed is weight divided by the lift/drag ratio. So a glider of weight 30pounds with a l/d ratio of 30:1 at 20knots needs 1pound of thrust to maintain level flight at 20 knots. But an F-104 weighing 30pounds with an l/d ratio of 3:1 at 80knots needs 10lbs of thrust to maintain level flight at 80knots. Both weigh the same but have totally different thrust requirements for level flight.

I have a Sabre that weighs just over 22 pounds at take off, powered by a J600 of 12lbs thrust but possibly losing a lot of thrust in the jet pipe, and it performs very well.