Justin, you can do a thread search and dig up quite alot of information on this and the Seagull version. You will find that many have used up to a .91 four stroke and always have the typical "insane power" and "unlimited vertical" comments but they always have to add wight to the tail to get it to balance. There was a recent thread from a fellow who installed an OS 55 AX and that worked extremely well. Suggestion here would be to try and keep the model light. I put one together with a 46AX and had a ball with it and it balanced perfectly and had a ton of power for anything you wanted to do except the "unlimited vertical". The weight of the ST 75 would make you add weight to balance although you would have more than enough power. The other thing about adding too large an engine on this model is that prop torque or "P" factor makes the plane a real bear to handle on the ground during take-off runs. It was already a handful with the smaller engine.