RE: CG Ultimate or GP Super Skybolt?
Compared to most bipe kits out there, the Goldberg Ultimate is a very easy build. I'm doing a GP Super Aeromaster right now; THAT's work. And I agree with the consensus; the Ultimate is hands-down the BEST-flying bipe out there. Mine's been hanging up for a while, sans engine and servos.
A YS 1.40 is over-kill, unless you demand being able to pull out of a hover at Warp 9, etc. For normal flying, a 1.20 four stroke is plenty. My first one had an OS 1.20; the current one used my Laser 150. Both were excellent. I agree that a 91 four stroke is a bit light. I know of one flying with an OS 91 Surpass that does okay; but the builder is a pro who went to great pains to lighten the structure. Anything over ten pounds is too heavy for this model, regardless of what engine used. My current one is a tad over nine pounds, and lands like a trainer.
Enjoy; this is a classic. I will definitely build another one before its all over...