From what I read I understand the YS 110 don't got a pump. The tank is pressurised. The 110 is supercharged. I have a YS 63 - no pump, pressurised fuel sytem.

A 2 stroke with 0.9 cubic inch displacement shoud easily develop a little more than 10 lbs thrust. Your 50x should weigh less than 5 lbs. 10 lbs thrust will pull a 5 lb plane straight up with 1 g acceleration until the plane gets going fast enough to develop a lot of drag. It sounds like your engine should do the job but isn't running very well.