For the tires, the 18x11-10 mean 18in diameter, by 11 in wide, by a 10in diameter rim. The width of the rim is not specified by the tire, but in general you would run 2-3 inches narrower on the rim width than the tire width. So, a 11in wide tire would run a 8-9in wide rim.
I am almost positive that the "1 mm" on the piston denotes an overbore dimension.
You can try service honda (www.servicehonda.com) for a set of rings, or partsfish.com, but you are going to have to pay shipping. Also, don't forget to get larger rings than stock. You should also be able to drive to your local dealer and buy them right there.
You may want to try and find some suiper scoops by Maier. They were made for the 350X, and may be available for the 200X. I am not sure if they still make them, but search online and you might find out.
I ran 18" turf tamers on my 200X and it did fine with a 1 tooth larger front sprocket. You may want to raise your front gear by 1 tooth, but leave it alone otherwise.
Joel Morris
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Purdue University Graduate Student
Mechanical Engineering Technology
1985 ATC70
web.ics.purdue.edu/~jsmorris