The brake disc

Machining ("turning") the rotors (discs) on an automotive brake job is fairly routine. This should not be done on a motorcycle. The rotors are much thinner, and you'll either end up with a rotor that's too thin for specs or warped from the heat. Rotors are extremely important for your safety. Replacing, rather than trying to fix it on the cheap, is the way to go if you have a problem one.

Rotor runout (warpage) is measured with your motorcycle. Lightly squeeze the brake lever at 15 mph. If you feel pulsing, the runout is too great and the rotor must be replaced.

Grooves and scarring are normal for a motorcycle rotor and shouldn't be a concern.

The minimum thickness is stamped right on the rotor. You can get a good idea of your thickness by checking it with your hand. Once the rotor "lip" gets extremely pronounced, it's probably time for replacement. Double-check with vernier calipers.

If you need to replace your rotor, there are usually plenty of lightly-used ones on eBay. Aftermarket rotors aren't necessarily better, and EBC ones in particular are of inferior quality. They tend to rust rather quickly, YMMV.