Hi MT,
when you take the cylinder head off the engine, hopefully there should be some faint writing visible (it will be very small). This will tell you what size piston you have fitted, if it says STD this means Standard, ie, the original size piston, this is very good news.
If it says 020, this means the cylinders have been bored out by 0.020 of an inch, and slightly larger pistons and rings have been fitted, this is what O/S refers to (Over-Size). This may have been done because there was a groove found in the cylinder bore, or just the wear was more than an acceptable level.
If it says '040' there is generally no more room for boring out. If there is wear or damage you dont have too many options; New STD cylinder block with STD pistons and rings is needed, or have a pair of liners fitted and bored out back to STD size (I recently had to do this second option). If the damage is only very light you may want to try just honing the cylinders, often a glazed cylinder bore can cause excessive exhaust smoke/high oil consumption.
Something to check, on some smaller engines like this, there may be non genuine 010 and 030 pistons and rings available, best to do some research.
Hope this helps!