I used the .61 and it had no difficulty with flying this plane (w/flaps and retracts). I'm sure the .91 would do just fine, IF it is a reliable engine.
Just had a thought....if you don't use the OS, make sure your muffler will clear the fuselage. My OS 61FX required a muffler extension to get it away from the fuse side.
Don't know about the ST....but check it out first.