The total weight listed for a 1.25 is 24.69 ounces, the weight for a saito 1.80 is 31 oz. 6-7 oz difference, not 2 oz. www.saitoengines.com to see for yourself. Not to mention it is dead weight because you will not have use for that power, the 1.25 is virtually unlimited vertical for it. So you can slap a 1.80 on it if you want, but your adding weight for no reason to a bring a 63" plane over the 9 lb mark. Look around, all the 3D planes in the 9-10lb range are the 67"+ birds....

As for mounting inverted or sideways, I understand that saito's will run inverted just fine, I have done so on many occasions. But the fact is that the gas still runs to the top of the head whenever you are not running the engine. Over time this causes gumming and build-up, which can cause.... you get the point.

Just making a suggestion as someone who has flown it and many birds in this class, the 1.80 will work great, but is definitely not needed.