It depends on what of performance you want. I've had Big Sticks and Ultra Sticks. On one I had a Mag .91FS. It it, in my opinion, not enough. A buddy had one with an OS .61FX and it was OK. I think the ST .75 is the best choice.

My present Ultra Stick 60 has a Tower .75 and a friend's Big Stick has a GMS .76. Both have plenty of power.

I do recommend you install everything except the rudder and elevator servos and check the CG. Sticks tend to come out nose heavy. I usually end up mounting those 2 servos in the rear, the rudder servo on top the fuselage and the elevator servo on the side.