Water, 20 over wouldnt be the last overbore. Thats only like .5mm not 2mm (80 over would be the 88mm)
BMH, I'd wait until you get it to see what the cylinder looks like before you buy a piston. If its in good shape you could just give it a hone and put the same size piston back in, if not you can bore it and put a bigger one in. (that is, if it is sleeved)