You should have no problem with the next size up piston. I always run a "B" Vertex in my "A" cylinder. I never used an Athena. Verify the size of the Athena "B" piston to be .01mm over the stock "A" piston, don't just go by the letter marking.
Also, unless you have a very good bore micrometer(expensive), don't worry about measuring the cylinder yourself. If you suspect wear then take it to a machine shop. The errors that will occur from improper measurement are greater than the differences in bore dia. that you'll be looking for.
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