When would a 401k loan default be due with the penalty and income tax to the remaining amount of the loan owed? Is it next year when taxes are filed?

[Critter]

If the default happens in 2017 then it is reported on the 2017 return ... in January you should get a 1099-R showing the distribution with a code 1 in box 7.  Since there will be no taxes withheld from the loan default you may need to make estimated tax payments or increase your withholding somewhere else to compensate.