Tearing into an engine is really easy, mark your parts as you remove them making note of placement, it will be down to bare block before you know it. The trick is all that measuring that requires precision tools, and knowing what cause failures, and what failures have done to other parts. Those marks look like something once got into the cylinder, maybe during a tune up, or a valve cover gasket job. One very obvious side affect of overheating is warped cylinder heads, but not so common knowledge is that the piston also swell and can scratch the cylinder walls or even seize midway in the cylinder. The cylinder can also warp, in very extreme cases I've seen blocks warp. Machine shop charges aren't terrible, a few hundred bucks to get the block decked, and cylinders honed, or perhaps bored out, and of course they will clean it up and it will look brand new. Some people just throw a used engine in, but wheres the leaning there, and what are the chances you put in that used motor only to find it's bad. Tear yours down, learn whats bad, if you do get a used unit I suggest pulling the oil pan and replacing the main and rod bearings, they aren't expensive.