yz125 has a plated cylinder. If the plating is worn enough for a bigger piston, then you'd need to replate anyway.
In other words, stick with the 54 mm piston. If your ring gap is wrong with new piston, or you have any bare aluminum showing through, or deep grooves, you'll want to send it out for replating.
There are some people who will get into the A,B,C piston sizing where you can get a 54.01 mm piston, etc. ... and there is some logic to this, but I've never bothered, and never had an issue.