What Brand?

It's a Schnuerle ported Fox .15 plain bearing. These engines while not rare, they're not commonly found either. This engine was made for a event that required plain bushing engines. The Fox .15 ball bearing version is much more common.  This engine in ball bearing form is pretty hot. Iron piston engine which needs a bit more castor over standard r/c fuel. Likes props from 7x4-7x6. The muffler is very restrictive and can cause overheating when pushed. If your looking to run the engine, remove the backplate and use caution not to tear the gasket. I do this by liberally letting oil soak around the perimeter and heat it a bit. Once off, insure that the rod is moving free back and forth on the wrist pin. No need to take this engine apart and I discourage that due to that nothing indexes the cylinder liner fit. Get behind the rod using a allen key and make certain it's sliding back and forth on the top end. Use heat, oil, whatever it takes to get it moving freely.  This engine uses wrist pin clips to hold the wrist pin into the piston. If the wrist pin and top of rod joint aren't freed up, the clips can come out while running. This can be instantly catastrophic.  I also caution that you don't lose the screws. Some of the screws in that engine are 3-48 screws which are not commonly found. Duke had surplus screws from the military and used this thread extensively throughout the years.