Is this paid or an award? If paid you could see about using DL codeshare on VA to help change the connection time. There's of course one SYD-LAX nonstop, but there's also options going SYD-BNE/MEL-LAX. If it's an award though you'd have to have availability at low level for the VA flights. That only happens when flight is first loaded and very close in to departure.

Of course can always take a later LAX-JFK flight or even go LAX-SFO-JFK.