It is an ABN engine, and Castor should be used. Even Morgan fuels does not recommend Cool Power for these types of motors. I would suggest Omega, and live with a little goo on the plane. The pitss will help keep most of it off the bottom to begin with.

The AX is a very good engine, you could probably get away without breaking it in on the bench, but its a simple process and worth doing. Personally, I'd break it in with the muffler you intend to use on the plane (the pitts), but using the stock muffler won't hurt anything.