Hi. In my humble opinion, the OS 1.20 AX probably won't need a pump. I am using that engine in my Excelleron with the stock tank position and get plenty of fuel flow from muffler pressure.... that is unless you decide to put the tank back near the horizontal stab..ha.. just kidding. That engine does produce plenty of power. I am using a 15-10 APC prop and it climbs straight up without effort.

DS.