Not familiar with the HP, but the OS 70 will fly it just fine. As for inverted, just make sure the tank is even with the spray bar in the carb, and you might want to consider a remote glow setup so you dont have to reach under the cowl to remove the glow starter with the engine running.
With the inverted engine the carb will be sitting lower than when upright, so you may need to lower the tank floor. On this plane, you might want to put the engine horizontal if you dont want to do any carving up the tank floor.