Maybe I'm coming from out in left field but I do know people that started out with a 91 4 cycle in the Ultimate then went to a 120. After flying with the 120's went back to the 91's because they liked the way it handled better with the lighter engine. Now weather they corrected the balance I don't know, their planes could have been to nose heavy.
The Goldberg is not as big as the other 120 Ultimates on the market and if you could keep it's weight around 7 1/2 lbs. it should be fine with a 91, great sport flying biplane. I will let you know this summer I'm going to finish mine (I hope), I'm planning on going with a YS 91 (about the same weight and power as the OS 120 so what was my point)???