Redcatcher,
I am in the final stages of completing my CG Ultimate (basically need to balance and set up the throws and have some nice weather). My Ultimate has a Saito 100 four stroke in it and that was per Goldberg Models tech support recommendation - they said to keep it light.

I have read threads on this site and others who fly this plane with four strokes up to a Saito 180! One of my flying buddies has an OS 91 FX two stroke in his and it flies great. I haven't put mine on the scale yet, but it feels really light - I'll let you know what it weighs after I put it on the scale.

SearchMan