Clean the plugs, check the gap and reinstall. Severe detonation, preignition, or over advance ignition timing can leave aluminum deposits from the pistons around the plug electrode. This indicates extensive internal engine damage. Preignition can also severely erode the plug end giving it a melted appearance. If the electrode appears to be dished then check the coil polarity and make sure the wires are attached according to your tractors specs.
Squashed electrode ends result from a mismatched plug.
The plug is probably too long and the piston has actually hit the end of it. When installing used or new plugs it is important to check the plug gap. This is the distance between the two electrodes at the base of the plug. If putting in a used plug check the electrodes for cleaness and squareness. A small ignition file should be used to break any oxidation off the ends.