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Any deviation from the example antenna could have significant and unintuitive consequences.  Unless you have good antenna modeling software and good understanding of antenna physics (which you apparently don't since you're asking here), don't deviate from the suggested design.  This includes the exact PC board material and thickness.

Even with the right software and understanding, it's an iterative process.  All real world details can't be modeled, and there are variations anyway.  Modeling can get you some good ideas to try, but in the end only experimental results can tell you what the antenna really does.

Also, as @Lundin alluded to in a comment, if you are making your own antenna, then compliance testing gets more complicated and expensive.  This is one reason whole modules are popular when it comes to RF transmitters.  The module manufacturer has already gotten certification for the module as a whole.  As long as you follow whatever use restrictions they specify, you don't need to get as much certification yourself.

The short answer for anyone who needs to ask this kind of question on the 'net is to <b>go buy a pre-certified module</b>.