I rebuilt a Sig Smith in the 90's. Originally it had a FOX 45 in the nose and it really screamed. I put a K&B 45 in it and it flew very nice and not underpowered at all. Since then I have substituted an XL52RFS four stroke. You can see my plane at my profile/models.
If you put a .60 in this plane I would really recommend a good check of the control surfaces, since it will be easy to get this model to flutter. It's gonna be a bullet.

Good Luck!

Charlie