I absolutely loved my Goldberg ARF but dumb thumbed it in. I was planning to put a Saito 125 in it before the Stacking. It had an OS .91 FS.
As for the engine, OS is expesive and not worth it. I favor Saito. Bolt it up run the fule lines and go. My OS .91 with the pump is a Pain in the ***. It is a good thing I did not pay pay for that engine fully as it is really not worth it. With the Saito engines they are simple, not a lot of plumbing.