RE: Goldberg Chipmunk with Saito 120 - advise
I have the World Models Arf Chipmunk (which is a clone of the Goldberg model). I'm running a Super Tiger .90 two stroke which would be roughly equivelent to your Saito 1.20.
I love it, and the plane seems to like it also. Just stick with a high diameter low pitch prop and you should be fine. I'm running an APC 15x4 Wide, but you could probably go up to a 16x4 with no problem.
I knew it was a clone, when I noticed the Carl Goldberg Logos on the corner of the decal set that came with the World Arf.