Not much you can do with them, even with a fully equipped electronics shack [Oscilloscope, surface mount soldering kit, micrometer and the circuit diagram ] the beam focus and tracking are crucial. These can be altered on some drives but to be honest the time it would take it's cheaper to bin the old one and buy new.
Wear and tear on the disc mount can alter the platter hieght throwing off the alignment. In a dire emergency you could try placing a 'shim' under the disc but don't hold breath.
Incidently if you have had a disc shatter in the drive odds are it's toast [both the disc and drive, but sometimes they will still read another CD]
PS: For those interested in learning or needing help with electronics google for 'Electro Tech Online'