Re. measurement of cylinders, it's not that simple. If the top of the jug is lipped from wear it has to be bored. If the cylinder is pitted it has to be bored. If the bores are in good shape it's possible they can be honed. Let a machine shop evaluate and measure the bores for you. You may be able to use your current pistons and maybe not.
If boring is required the shop will be able to tell you how much material they'd have to remove to get the bores clean, and based on that you'd select the appropriate oversize, obtain the pistons, bring the pistons to the shop, and then have the cylinders bored to them. Piston oversizes are specified in increments of .025 mm.=.010". If they tell you, for example, "We'll need to go twenty over," that means you'll need a piston .020" larger than what's in place--2nd oversize, if your current pistons are original.