OS makes a ridiculous recommend prop selections for this engine.
Recommended Props: 12x7-8, 13x6-7
As you increase the load on an engine, the acceptible needle range that the engine will run with gets to zero. Either their marketing department thinks that they will sell more engines if people compare the recommended prop size (ie., if this one turns bigger props, it must be more powerful), or they are trying to make a very quiet engine. Either way, those props are way too much load.
Depending on the model, I would try either a 12x6 or 11x7 as a starting point. Shoot for an rpm of at least 11,000. The engine is really just a pumped up .45, so it may not flow very well internally.