just take the barrel some where and get it re-bored over size, piston £20-£100 re-bore is about £25 thats if the bore has enough left in the sleeve to go up a size
no point using a 2nd hand barrel, if ur rebuilding the top end may aswell have a nice fresh top end
If its cracked get one and have it bored,chamfered,and honed to the exact tolerances of the new piston you already bought,can recommend mitaka pistons and as Norfe said "fresh top end ",make sure you use a yammy head gasket and your laughing
