Maximum safe bore on any motor will be determined by sonic checking of the cylinder walls. You need 0.135" thickness after boring, honing, etc. You could determine how much the head could be milled by removing the head and stacking some modeling clay on the valves, then bolting the head back on and turning the motor over with a socket/handle. Then remove the head and measure the thickness of the clay. I'd say that if you had 0.060" on the intakes and 0.080" on the exhausts clearance between the valves and the head, you'd be ok.