Mayday,

You must remember that your new engine is not a ringed engine.
It must not be broken-in cold, but at working temperature, that will cause the sleeve to expand.

It must be run rich, with plenty of Castor oil in the fuel (I recommend 21-22% total oil for break-in), but not rich enough to cause it to four-cycle. About 1/2-3/4 of a turn richer than peak is OK and finding this peak, in an ABC/ABN engine, will not cause any damage. Don't bother to touch the idle needle, before completing the break-in and running these 15-30 minutes on a test stand will be much better for the engine, for you and for the plane.

Starting after 15 minutes, peak the engine and listen if it can hold peak without ANY speed reduction, for 60 seconds.
If it can, you are done. If not, richen it immediately and continue the break-in for 5 more minutes and try this again.

Running it in four-cycling and cold will potentially damage it permanently.

Please go to the OS support forum and see what Bill Baxter has to say about break-in of this type of engines.

An 11x7.5, or an 11x8 prop, will give it a light load and plenty of cooling air.