If you already have a Saito .91, use it. It's light and powerful and wouldn't take as much moving stuff to the rear to get the CG correct.

I am flying an Ultra Stick 60 with an SK .91 and a buddy has the Big Stick 60. He had a Super Tigre .91 in it until the bearings went, now he has a GMS .76 in it. Both pull it well.

I also have a couple of Saito .91s and a Magnum .91FS. The 4-strokes will make it a nice flying plane, maybe a little better than a 2-stroke .61. To wake it up you need a good .75 or a .91 2-stroke or a 1.20 4-stroke.