I don't think any WWI aircraft has anywhere near 1:1 power-weight ratio. Probably not even 1:5. I don't know which WWI aircraft you're modeling, but 163.4" wingspan is at least 30% of the real thing. At that scale, it will likely have flight characteristics very similar to full scale. I think a couple of Ryobi 31cc will give a good measure of reserve power. At the quoted 26lbs of combined thrust, that'll yield a 1:2 power-weight ratio. Not at all bad for a WWI vintage twin.