I run some weedeater type motors in RC helicopters and NGK makes some plugs with iridium elements and they are by far the best I've used. Don't know the number for the plug in our bike engines, but I'm sure NGK would have a cross reference.
If you take a look at the length of the stock plug with the head off (plug screwed in), you might be surprised. The stock plug is slightly too short. To correct this problem, try a NGK B6HS or ND W20FS-U plug. These are high quality plugs compared to the stock plug.
Hello from Spain, I bought my bike two weeks ago and I saw that de spark plug is very bad. I have changed the spark plug for this one: NGK BS7ES. This plug is suitable for piaggio and derbi mopeds and is the best for chinesse bike engines.
I'm sorry for my little English.