A bore gage is necessary to accurately measure the wear of engine cylinders.
First, determine the bore diameter you are trying to measure. in the case of the Cub, it is 2.625 to 2.627 (2 5/8") Set a micrometer to that dimension and lock the dial.
Install the correct length adapter on the gage so that you can 'set' the dial indicator on '0' which equals 2.625 inches. your eyes are not deceiving you, this example is using a 3.625 inch bore on a John Deere combine engine.
measure each cylinder at 90 degrees to the wrist pin in 3 places top, middle, and bottom of ring travel in the cylinder. Repeat the procedure measuring in the direction of the wrist pin. this is to indicate the amount of 'out of round' in the cylinders. This picture, although a little fuzzy, shows .005 on the gage. Remember we set '0' on the gage to be 3.625, so the gage is reading the wear of the cylinder from nominal.
You will ordinarily see the most wear at the top of the cylinder where the most thrust is. Most engine specifications will allow .005 to .008 wear before saying the cylinder is 'out of spec'.
Now comes the subjective part. Boring an engine involves completely tearing it down until the camshaft sits on the bench, freeze plugs and bearings removed, if any. It also involves about $350 at the shop and 4 new pistons. So the decision to bore is one that has to balance your needs for the engine and your pocketbook. Scoreing, rust pitting, and sand scouring (when the air cleaner does not get hooked up), all play a part in the decision to bore.
The choice is never easy. In addition, good running Cub motors are available. So if you don't need that casting code to match, $500 to $600 can buy you a nice running engine. Or a used 'long block' for half that.