you need the standard size! .25 is 1/4mm larger .50 is 1/2mm larger etc,that requires the bores to be rebored over size by that amount and new pistons aswell,you can get away with murder in these engines if the bore is in spec,new rings will need the bore honed,this takes off that shiney glaze and leaves it somewhat scratched looking,those scratches hold oil as the new rings grind(for want of a better word,like bed in?) into the fresh surface and create a good seal,be sure to clean the pistons well,by hand run a 1mm drill bit through the oiling holes and get any and all carbon out of the ring grooves,you can snap an old ring and grind it to make a scraping tool for those grooves.
