I would suggest sticking with a larger motor. Hopping up a given displacement motor usually brings out the horsepower at higher RPM on the same size prop. Biplanes are draggy and as a rule do better with larger props. A short nosed biplane often will need more nose weight when fitted with a lighter weight engine. I understand wanting to use an engine that you have on hand. The .45 will probably get the Pup off the ground but performance will be sub optimal.