It is a bit complicated to explain, but normally if you increase bore size, even if you use a perfectly flat piston, you can increase compression a little, but normally thats not how it is done.

The easiest way is to install a higher compression piston and the dome on top of the piston is larger, resulting in it compression the air above the piston more than a piston with a lower dome.

You also can increase compression by skimming the head or barrel.

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Disclaimer: any mod undertaken, are carried out at owners risk