I haven't seen a nylon plug in a generator, but I have had this problem in an Atwood water heater. I flush my WH 3 or 4 times a year and NEVER re-use a nylon plug more than once as they do get hard/brittle from the heat. The WH plugs are hollow, and when they break the contents of the WH drain. Then the threads still in the heater can be removed with anything that will grip them. I have used a hunting knife that I can push into the nylon and then turn with the help of a pair of pliers.
If the plug sheared off and the contents have NOT drained, I would drill hole into the plug to drain the oil out, then use an easy-out, knife, or anything else that would grip the nylon.
Paul (KE5LXU) ...was fulltimin', now parttimin'
'03 Winnebago UA 40e / '05 Honda Odyssey toad