The JG is a TM clone (same as the Cyma) so as far as internals go, standard hop rubber should fit fine but be aware that ones with a longer feed lip (like the Maple Leaf ones) may protrude into the feed tube a bit.
As for barrels, just make sure it has both sets of grooves in the side of the barrel, the hop unit clip goes in a different place than on an M4. ZCI ones will work, or if you want to spunk a bunch more cash, Prometheus do an M14 specific one. You're looking for something around 420mm length. Worth mentioning though that if the cylinder is the same sort of size as a TM or Cyma, changing to a tighter bore will push your FPS up a not inconsiderable amount!