About the biggest you can go oversize is 1 mm and find a piston that will fit. This will make a negligible difference in power.

You might stay with the original bore size and maybe you can use the same pistons. Pistons are very expensive for these engines.

If the engine was run out of oil, the crank shaft may not be regrindable. I think these cranks are nitrided so if you regrind it, you should renitride it.

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