The stock nozzle has an overall length of 11.5mm.
Threads are M6x1.0 with a 6.5mm length.
Nozzle opening diameter is 0.4mm
(For reference a genuine E3D nozzle has a thread length of 7.5mm)
GiGDigit - Nozzle - 0.4mm - (Qty 2)
Many nozzles with M6 threads should work but will likely require adjustments.