It really depends on what type of Jeet they're studying. After Sifu Lee's death, two major "schools" of JKD sprang up. One branch took to heart Bruce's idea that Jeet was a Way Without A Way, and kept it evolving, kept borrowing, kept adding to it. They went by Bruce's philosophies, kept it "like water," formless and powerful, and have been adding to it ever since. The other school of thought was that JFK was Bruce Lee's invention, Bruce Lee's baby, and they've done their best to leave it like
he left it. They're the sorts that are more likely to still be fighting in a style recognizable as Bruce's.
For a classic JKD type like that, you could probably just use the rules for Kung Fu (a hair more realistic) or Tae Kwon Do (Bruce wasn't a fan of big flashy kicks in real life, but he used them an awful lot in the movies, so for a more cinematic JKD, Tao Kwon Do works). Either one would leave you with a nice "Eastern" looking flurry of Bruce Lee straight-blasts, cool jumping dramatic kick charges, and plenty of opportunities to let out alleycat howls while breaking people's ribs.
For a more open Jeet practitioner, use anything. Yes, really. The entire idea behind the art is "use what works." Western Boxing, Muay Thai knees and elbows, Wing Chun handwork, borrowed strikes that Guro Dan Inosanto brought into JKD from the Filipino arts, some schools have starting to incorporate some ground fighting... you name it. I imagine that, sixty years from now, JKD guys will be incorporating moves from Wildcat and Firefight and everything else they can get their hands on. We're cool like that.