I can help on the piston. The arrow should be facing forward when installed in the engine towards the front of the bike. This insures that the piston rings do not hang up on any of the ports in the cylinder jug.
As far as motor identification I feel like I am loosing track. The smaller china has a shorter stroke and the bigger china a longer one. The smaller china will rev more the bigger china seams powerful all the way around from what I hear. http://motorbicycling.com/f52/basics...ing-16646.html
This might help shed light on the subject. Consider getting a tuned pipe too.